Remove deprecated decorator
Remove deprecated decorator

Should use the normal Flask error handler. Making this change gets rid
of a deprecation warning from our logs.

gov-cjwaszczuk/notifications-admin,gov-cjwaszczuk/notifications-admin,gov-cjwaszczuk/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,gov-cjwaszczuk/notifications-admin