osx: let qcombobox specify target rect when showing popup

Let qcombobox use the new API for showing a native
popup menu (introduced with 1a47595).

Change-Id: Id08ef9e59fdd47b2c1df84fa72e3a2c69fe187b0
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
qplatformmenu: remove unused and deprecated function

Now that all platforms use the popup function that
takes a target rect, lets remove the deprecated one.

This is sort of important, since QtQuick controls now
uses the new version. So if a (new) platform ends up only
implementing the old version, that can end up not
working correctly.

Change-Id: I34814b3de5ea4954cf21b161e8a834e39e5534c8
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>