Remove workaround for unsupported compiler

Qt for WinRT now requires Visual Studio 2015.

Change-Id: I8996f2855476315819532df89c6754aa162bd02e
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>