Remove finfo class existence checks

As of PHP >= 5.3.0 finfo class is available [1] and enabled by default [2]

[1] http://www.php.net/manual/en/class.finfo.php
[2] http://www.php.net/manual/en/fileinfo.installation.php

Removed all runtime checks but kept them in install/check to have an easy way to find the case where someone disabled the extension.

Issue 0023639