Don't use qt_mac_get_fixed_pitch() to resolve fixed pitch fonts on OSX

The call was resulting in inifinite recursion on OSX 10.9 when Qt was
built against the 10.7 SDK, as qt_mac_get_fixed_pitch uses QFontMetrics
to resolve the pitch, and we would end up in the font resolver again
when asking for the metrics.

The CoreText font-database already takes care of resolving whether or
not a font family is fixed-pitch, so the code is likely a leftover
from the ATSUI-days, and can be removed.

Task-number: QTBUG-31803
Change-Id: I37c90fa637927eb4adc16c0fd556c4c46c456034
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>