Remove the MacMainWindow example.

Remove the MacMainWindow example.

setUnifiedTitleAndToolBarOnMac will not be supported
on Qt 5, which removes the point of having this example.

The replacement for setUnifiedTitleAndToolBarOnMac
will be a NSToolBar wrapper in the MacExtras module,
and this example will probably re-appear in some form
there.

Change-Id: I46d5e70a7c45625d9334911bb48059e86f05282d
Reviewed-by: Christoph Schleifenbaum <christoph.schleifenbaum@kdab.com>
Reviewed-by: Bradley T. Hughes <bradley.hughes@nokia.com>