www/firefox: drop broken GTK2 option following upstream

https://www.mail-archive.com/dev-platform@lists.mozilla.org/msg20523.html

PR: 218569
Reported by: Walter Schwarzenfeld, Jakub Lach <jakub_lach@mailplus.pl>
Submitted by: rezny (based on)
MFH: 2017Q2 (r436907 followup)