Remove dead mime_content_type() code paths

Since PHP 5.3, mime_content_type() is implemented as part of PHP's
fileinfo extension and thus is deprecated in favor of the newer
alternatives provided by that extension (already in use).

Also updated a comment in DefaultSettings.php that mentioned the
function. Did not modify lessc.inc.php, a third-party library
maintained separately from MediaWiki.

Change-Id: Ic4a0873989ddb634ec9a05c3340941a9ba3f5ec5
Merge "Remove dead mime_content_type() code paths"