gecko: drop poorly maintained GIO=off case after r376999

Glib IO is already used by Gtk internally, so there's little reason
to avoid it unless using non-Gtk builds which are no longer supported.

https://bugzilla.mozilla.org/show_bug.cgi?id=1286627
https://bugzilla.mozilla.org/show_bug.cgi?id=1287288

PR: 210220
Reported by: Marco Beishuizen, Arrigo Marchiori
MFH: 2016Q3