Remove mark on project_ci_cd_settings.merge_trains_enabled

Closes https://gitlab.com/gitlab-org/gitlab/issues/36461

See merge request https://gitlab.com/gitlab-org/gitlab/merge_requests/23147

app/models/project_ci_cd_setting.rb

 # frozen_string_literal: true
 
 class ProjectCiCdSetting < ApplicationRecord
-  include IgnorableColumns
-  # https://gitlab.com/gitlab-org/gitlab/issues/36651
-  ignore_column :merge_trains_enabled, remove_with: '12.7', remove_after: '2019-12-22'
   belongs_to :project, inverse_of: :ci_cd_settings
 
   # The version of the schema that first introduced this model/table.