Removes unnecessary cop on migration

Migration/AddColumnWithDefault cop was deprecated on
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/31736, so we don't
need to disable it anymore.

Fixes https://gitlab.com/gitlab-org/gitlab/-/issues/217968

db/migrate/20200320212400_add_project_show_default_award_emojis.rb

@@ -6,6 +6,6 @@ class AddProjectShowDefaultAwardEmojis < ActiveRecord::Migration[6.0]
   DOWNTIME = false
 
   def change
-    add_column :project_settings, :show_default_award_emojis, :boolean, default: true, null: false # rubocop: disable Migration/AddColumn
+    add_column :project_settings, :show_default_award_emojis, :boolean, default: true, null: false
   end
 end