Bug 1638193 - Remove build system support for iOS. r=nalexander

iOS support for Gecko has not been tested in years and is most probably
out of date. The build system part of it, specifically the checks in
build/autoconf/ios.m4, are not trivial to port to python configure, and
they prevent other things from moving to python configure (because some
of them change value when MOZ_IOS is set).

The code is left alone, although it could probably be stripped off as
well, but I'll leave that as an exercise for someone else.

Differential Revision: https://phabricator.services.mozilla.com/D75463