Remove `lock_version` from the `merge_requests` table.

- This was introduced (at some point) while rebasing `master` against
  the "developers can merge" feature branch.

- https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5310#note_13139705

db/schema.rb

@@ -624,7 +624,6 @@ ActiveRecord::Schema.define(version: 20160712171823) do
     t.integer  "merge_user_id"
     t.string   "merge_commit_sha"
     t.datetime "deleted_at"
-    t.integer  "lock_version",              default: 0,     null: false
     t.string   "in_progress_merge_commit_sha"
   end
   add_index "merge_requests", ["assignee_id"], name: "index_merge_requests_on_assignee_id", using: :btree