PHPMailer: Remove a check for PHP 5.0

Summary:
Phorge requires PHP 5.5, thus remove a version check for PHP 5.0.
Compare the same code in https://github.com/PHPMailer/PHPMailer/blob/master/src/POP3.php#L282-L283

Test Plan: Carefully read the code.

Reviewers: O1 Blessed Committers, speck

Reviewed By: O1 Blessed Committers, speck

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25661