Remove unused strings EXTENSION_CONTROLLER_MISSING and EXTENSION_CLASS_WRONG_TYPE from common.php

	'EXTENSION_CONTROLLER_MISSING'	=> 'The extension <strong>%s</strong> is missing a controller class and cannot be accessed through the front-end.',
'EXTENSION_CLASS_WRONG_TYPE'	=> 'The extension controller class <strong>%s</strong> is not an instance of the phpbb_extension_controller_interface.',