[ticket/12455] Remove unused language strings from common.php and app.php

Removed the following unused language strings from common.php:
EXTENSION_CONTROLLER_MISSING
EXTENSION_CLASS_WRONG_TYPE

Removed the following unused language strings from app.php:
CONTROLLER_NOT_FOUND
CONTROLLER_SERVICE_NOT_GIVEN
CONTROLLER_RETURN_TYPE_INVALID

PHPBB3-12455