PHPMailer: Remove check for non-existing get_magic_quotes()

Summary: No idea if PHP ever offered such a method. In any case, this very check was removed in https://github.com/PHPMailer/PHPMailer/commit/6f919df65e80023fce4eab58832b6035b4a41935#diff-2d294eae28fb0d70fc32da8369c4fe33d301c99ca3eeb4bd41da18ad99344d68 in 2012.

Test Plan: Not sure - how to test a removed check for a method which does not exist anyway? :)

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25663