Remove reference to QMAKE_SUPPORTED_ORIENTATIONS

We decided to wait with support for setting the orientation in
the .pro file until we can agree on a cross-platform solution,
as we already have the Android-specific solution of setting it
in the manifest.

Task-number: QTBUG-34166
Change-Id: I63566888e85a4ede97f32aead961e8ac22d4945b
Reviewed-by: BogDan Vatra <bogdan@kde.org>