Master is broken due to missing assignee_id

Example: https://gitlab.com/gitlab-org/gitlab-ee/builds/17108650

Failures:

  1) Milestone draggable issues allows admin to drag issue
     Failure/Error: issuable.assign_attributes(params.merge(updated_by: current_user))

     ActiveRecord::UnknownAttributeError:
       unknown attribute 'assignee_id' for Issue.
     # ./app/services/issuable_base_service.rb:217:in `update'
     # ./app/services/issues/update_service.rb:8:in `execute'
     # ./app/controllers/projects/issues_controller.rb:135:in `update'
     # ./app/controllers/application_controller.rb:285:in `set_locale'
     # ./lib/gitlab/middleware/multipart.rb:93:in `call'
     # ./lib/gitlab/request_profiler/middleware.rb:14:in `call'
     # ./lib/gitlab/middleware/go.rb:16:in `call'
     # ./lib/gitlab/etag_caching/middleware.rb:10:in `call'
     # ./lib/gitlab/middleware/readonly_geo.rb:30:in `call'
     # ./lib/gitlab/request_context.rb:18:in `call'
     # ./config/initializers/fix_local_cache_middleware.rb:9:in `call'
     # ./lib/gitlab/middleware/static.rb:9:in `call'
     # ./lib/gitlab/testing/request_blocker_middleware.rb:36:in `call'
     # ------------------
     # --- Caused by: ---
     # NoMethodError:
     #   undefined method `assignee_id=' for #<Issue:0x000000248ce550>
     #   Did you mean?  assignee_ids=
     #                  assignee_ids
     #                  assignees=
     #                  assignee_list
     #   ./app/services/issuable_base_service.rb:217:in `update'

It appears https://gitlab.com/gitlab-org/gitlab-ee/commit/8c0ab358320680eb1507140d59e3766bccd98e9b dropped the column. I think we need to reconsider this migration because:

1. Downgrading will lose all assignees
2. Other specs haven't been updated

/cc: @vsizov