Revert "Merge branch '21801-remigrate-epic-user-mentions' into 'master'"
What does this MR do?
Reverts !25259 (merged) because of a statement timeout on post-deploy migrations:
https://ops.gitlab.net/gitlab-com/gl-infra/deployer/-/jobs/967836
== 20200214174607 RemigrateEpicNotesMentionsToDb: migrating ===================
-- transaction_open?()
-> 0.0000s
-- index_exists?(:notes, :id, {:where=>"note LIKE '%@%'::text AND notes.noteable_type = 'Epic'", :name=>"epic_mentions_temp_index", :algorithm=>:concurrently})
-> 0.0138s