Removes AddTimezoneToIssuesClosedAt migration and respective cleanup post migration for C

db/migrate/20171106171453_add_timezone_to_issues_closed_at.rb

-# See http://doc.gitlab.com/ce/development/migration_style_guide.html
-# for more information on how to write migrations for GitLab.
-
-class AddTimezoneToIssuesClosedAt < ActiveRecord::Migration
-  include Gitlab::Database::MigrationHelpers
-
-  # Set this constant to true if this migration requires downtime.
-  DOWNTIME = false
-
-  disable_ddl_transaction!
-
-  def up
-    change_column_type_concurrently(:issues, :closed_at, :datetime_with_timezone)
-  end
-
-  def down
-    cleanup_concurrent_column_type_change(:issues, :closed_at)
-  end
-end

db/post_migrate/20171106180641_cleanup_add_timezone_to_issues_closed_at.rb

-# See http://doc.gitlab.com/ce/development/migration_style_guide.html
-# for more information on how to write migrations for GitLab.
-
-class CleanupAddTimezoneToIssuesClosedAt < ActiveRecord::Migration
-  include Gitlab::Database::MigrationHelpers
-
-  DOWNTIME = false
-
-  disable_ddl_transaction!
-
-  def up
-    cleanup_concurrent_column_type_change(:issues, :closed_at)
-  end
-
-  # rubocop:disable Migration/Datetime
-  def down
-    change_column_type_concurrently(:issues, :closed_at, :datetime)
-  end
-end