Remove workarounds for older PHP versions

This patch removes the workarounds for PHP versions < 5.2.0 where `PATHINFO_FILENAME` support is missing. The CakePHP core already requires PHP 5.2.8 as a minimum, not to mention that this functionality is used in the `GeneratorBehaviour` class [since ages](https://github.com/davidpersson/media/commit/3dc0a99542f93e38f279f8dcd070639143cd4624#diff-c773dbc49ea9eafb6801aeea766ab820), so these workarounds should be be safe to drop.