Remove Temporary Index on Issues

What does this MR do?

Removes temporary index idx_on_issues_where_service_desk_reply_to_is_not_null used for a migration in 13.9

Migration Output

lee@cc-gdk-2:~/gitlab-development-kit/gitlab$ bundle exec rake db:migrate:up VERSION=20210216223335 RAILS_ENV=development
== 20210216223335 RemoveIndexOnIssuesWhereServiceDeskReplyToIsNotNull: migrating 
-- indexes(:issues)
   -> 0.0085s
-- transaction_open?()
   -> 0.0000s
-- indexes(:issues)
   -> 0.0071s
-- execute("SET statement_timeout TO 0")
   -> 0.0006s
-- remove_index(:issues, {:algorithm=>:concurrently, :name=>"idx_on_issues_where_service_desk_reply_to_is_not_null"})
   -> 0.0023s
-- execute("RESET ALL")
   -> 0.0004s
== 20210216223335 RemoveIndexOnIssuesWhereServiceDeskReplyToIsNotNull: migrated (0.0199s)

lee@cc-gdk-2:~/gitlab-development-kit/gitlab$ bundle exec rake db:migrate:down VERSION=20210216223335 RAILS_ENV=development
== 20210216223335 RemoveIndexOnIssuesWhereServiceDeskReplyToIsNotNull: reverting 
-- indexes(:issues)
   -> 0.0076s
-- current_schema()
   -> 0.0001s
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:issues, [:id], {:name=>"idx_on_issues_where_service_desk_reply_to_is_not_null", :where=>"service_desk_reply_to IS NOT NULL", :algorithm=>:concurrently})
   -> 0.0074s
-- execute("SET statement_timeout TO 0")
   -> 0.0004s
-- add_index(:issues, [:id], {:name=>"idx_on_issues_where_service_desk_reply_to_is_not_null", :where=>"service_desk_reply_to IS NOT NULL", :algorithm=>:concurrently})
   -> 0.0041s
-- execute("RESET ALL")
   -> 0.0004s
== 20210216223335 RemoveIndexOnIssuesWhereServiceDeskReplyToIsNotNull: reverted (0.0225s)

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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #321745 (closed)