remove obsolete migration

as we destroy all gpg_signatures we don't need to downcase the email
addresses anymore.

db/post_migrate/20170828124518_downcase_gpg_key_user_email_on_gpg_signatures.rb

-class DowncaseGpgKeyUserEmailOnGpgSignatures < ActiveRecord::Migration
-  DOWNTIME = false
-
-  include Gitlab::Database::MigrationHelpers
-  disable_ddl_transaction!
-
-  class GpgSignature < ActiveRecord::Base
-    self.table_name = 'gpg_signatures'
-
-    include EachBatch
-  end
-
-  def up
-    GpgSignature.each_batch do |relation|
-      relation.update_all('gpg_key_user_email = LOWER(gpg_key_user_email)')
-    end
-  end
-
-  def down
-    # we can't revert the downcasing, but actually we don't need to really, as
-    # downcasing the emails is not a harmful change.
-  end
-end