Remove column ignore for plan_id and trial_ends_on

Both columns have been ignored for at least 1 release

app/models/namespace.rb

@@ -13,9 +13,6 @@ class Namespace < ApplicationRecord
   include Gitlab::Utils::StrongMemoize
   include IgnorableColumns
 
-  ignore_column :plan_id, remove_with: '13.1', remove_after: '2020-06-22'
-  ignore_column :trial_ends_on, remove_with: '13.2', remove_after: '2020-07-22'
-
   # Prevent users from creating unreasonably deep level of nesting.
   # The number 20 was taken based on maximum nesting level of
   # Android repo (15) + some extra backup.