Remove Generic.PHP.ForbiddenFunctions sniff

This removes the check for the deprecated functions delete() and sizeof().
Those checks are currently choking with namespaced classes with these names.

Like: https://github.com/joomla/jissues/blob/master/src/App/GitHub/Controller/Ajax/Labels/Delete.php#L19

Example Travis log: https://travis-ci.org/joomla/jissues/jobs/14662814