Remove PHP detection from entry points other than index.php

Remove PHP detection from entry points other than index.php

The following entry points already didn't have it:

* opensearch.php
* thumb.php
* thumb_handlers.php

Now removed from api.php and load.php as well. The check was broken
because these entry points use the 'use' keyword for PHP namespaces,
which results in a syntax error on PHP < 5.3.

The check remains in index.php, mw-config/index.php, and
Maintenance (for e.g. install.php).

Bug: T112942
Change-Id: I9026d69ddfb3e02612256b4a9df525c432bb33c6