[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
Merge pull request #2401 from n-aleha/ticket/12455

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

* n-aleha/ticket/12455:
  [ticket/12455] Remove unused language strings from common.php and app.php
[ticket/12378] Prosilver common.css duplicate entry

https://tracker.phpbb.com/browse/PHPBB3-12378

PHPBB3-12378
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/12455] Remove unused language strings from common.php and app.php
Merge pull request #2402 from MGaetan89/ticket/12378

[ticket/12378] Prosilver common.css duplicate entry

* MGaetan89/ticket/12378:
  [ticket/12378] Prosilver common.css duplicate entry
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/12378] Prosilver common.css duplicate entry