Add clean-up phase for merge request assignees background migration

Follow up of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26496:

1. Execute existing on-going background migrations (https://docs.gitlab.com/ee/development/background_migrations.html#cleaning-up)
2. Ignore column MergeRequest#assignee_id
3. Deleted belongs_to :assignee from MergeRequest model
4. Delete feature flag