[Mac] Fix ref count bug when disposing windows
[Mac] Fix ref count bug when disposing windows

The fix introduced in 1af157a05698b084882519099e92a4f2b55d881c
is not required since Xamarin.Mac version 5.6 and has
the opposite effect.

Fixes VSTS #756962

antmicro/xwt,hwthomas/xwt,lytico/xwt,TheBrainTech/xwt,mono/xwt