Configure: don't enable the WMF multimedia backend by default.

This means we now use DirectShow as default multimedia backend
on Windows.

Task-number: QTBUG-45597
Change-Id: If95bbb8e7b33d73d80f7ba42de63ac54539e15b8
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>