Revert 28560 cleanup optimistic locking db pt2

What does this MR do?

Reverts two MRs:

• !25396 (merged) since it's causing a statement timeout on staging - https://ops.gitlab.net/gitlab-com/gl-infra/deployer/-/jobs/1113692
• !30018 (merged) - Includes a modification of the first MR

Log from gstg-postdeploy-migrations

fatal: [deploy-01-sv-gstg.c.gitlab-staging-1.internal]: FAILED! => changed=true 
   cmd:
   - /usr/bin/gitlab-rake
   - db:migrate
   delta: '0:21:52.800336'
   end: '2020-04-22 14:21:36.573211'
   msg: non-zero return code
   rc: 1
   start: '2020-04-22 13:59:43.772875'
   stderr: |-
     rake aborted!
     StandardError: An error has occurred, all later migrations canceled:
   
     PG::QueryCanceled: ERROR:  canceling statement due to statement timeout
     /opt/gitlab/embedded/service/gitlab-rails/app/models/concerns/each_batch.rb:78:in `block in each_batch'
     /opt/gitlab/embedded/service/gitlab-rails/app/models/concerns/each_batch.rb:68:in `step'
     /opt/gitlab/embedded/service/gitlab-rails/app/models/concerns/each_batch.rb:68:in `each_batch'
     /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/database/migration_helpers.rb:1095:in `queue_background_migration_jobs_by_range_at_intervals'
     /opt/gitlab/embedded/service/gitlab-rails/db/post_migrate/20200217210353_cleanup_optimistic_locking_nulls_pt2.rb:31:in `block in up'
     /opt/gitlab/embedded/service/gitlab-rails/db/post_migrate/20200217210353_cleanup_optimistic_locking_nulls_pt2.rb:28:in `each'
     /opt/gitlab/embedded/service/gitlab-rails/db/post_migrate/20200217210353_cleanup_optimistic_locking_nulls_pt2.rb:28:in `up'
     /opt/gitlab/embedded/bin/bundle:23:in `load'
     /opt/gitlab/embedded/bin/bundle:23:in `<main>'
   
     Caused by:
     ActiveRecord::QueryCanceled: PG::QueryCanceled: ERROR:  canceling statement due to statement timeout
     /opt/gitlab/embedded/service/gitlab-rails/app/models/concerns/each_batch.rb:78:in `block in each_batch'
     /opt/gitlab/embedded/service/gitlab-rails/app/models/concerns/each_batch.rb:68:in `step'
     /opt/gitlab/embedded/service/gitlab-rails/app/models/concerns/each_batch.rb:68:in `each_batch'
     /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/database/migration_helpers.rb:1095:in `queue_background_migration_jobs_by_range_at_intervals'
     /opt/gitlab/embedded/service/gitlab-rails/db/post_migrate/20200217210353_cleanup_optimistic_locking_nulls_pt2.rb:31:in `block in up'
     /opt/gitlab/embedded/service/gitlab-rails/db/post_migrate/20200217210353_cleanup_optimistic_locking_nulls_pt2.rb:28:in `each'
     /opt/gitlab/embedded/service/gitlab-rails/db/post_migrate/20200217210353_cleanup_optimistic_locking_nulls_pt2.rb:28:in `up'
     /opt/gitlab/embedded/bin/bundle:23:in `load'
     /opt/gitlab/embedded/bin/bundle:23:in `<main>'
   
     Caused by:
     PG::QueryCanceled: ERROR:  canceling statement due to statement timeout
     /opt/gitlab/embedded/service/gitlab-rails/app/models/concerns/each_batch.rb:78:in `block in each_batch'
     /opt/gitlab/embedded/service/gitlab-rails/app/models/concerns/each_batch.rb:68:in `step'
     /opt/gitlab/embedded/service/gitlab-rails/app/models/concerns/each_batch.rb:68:in `each_batch'
     /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/database/migration_helpers.rb:1095:in `queue_background_migration_jobs_by_range_at_intervals'
     /opt/gitlab/embedded/service/gitlab-rails/db/post_migrate/20200217210353_cleanup_optimistic_locking_nulls_pt2.rb:31:in `block in up'
     /opt/gitlab/embedded/service/gitlab-rails/db/post_migrate/20200217210353_cleanup_optimistic_locking_nulls_pt2.rb:28:in `each'
     /opt/gitlab/embedded/service/gitlab-rails/db/post_migrate/20200217210353_cleanup_optimistic_locking_nulls_pt2.rb:28:in `up'
     /opt/gitlab/embedded/bin/bundle:23:in `load'
     /opt/gitlab/embedded/bin/bundle:23:in `<main>'
     Tasks: TOP => db:migrate
     (See full trace by running task with --trace)
   stderr_lines: <omitted>
   stdout: |-
     == 20200217210353 CleanupOptimisticLockingNullsPt2: migrating =================
     -- transaction_open?()
        -> 0.0000s
     -- index_exists?(:ci_stages, :lock_version, {:where=>"lock_version IS NULL", :algorithm=>:concurrently})
        -> 0.0078s
     -- transaction_open?()
        -> 0.0000s
     -- index_exists?(:ci_builds, :lock_version, {:where=>"lock_version IS NULL", :algorithm=>:concurrently})
        -> 0.0203s
   stdout_lines: <omitted>

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• [-] Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done