Remove annoying log entry if function is not found

On some systems that blacklist some offending functions, there's a continuous CiviCRM log entry as such:

mailreader: Internal PHP function getmypid is not found or blacklisted.

As this is becoming tedious, we'll be removing this warning message.

For reference: If the function does not exist, we cannot process the $files variable so we need to bring that back empty.