winrt: Use SwitchToThread instead of Sleep(0)

UWP also supports SwitchToThread. The usage of Sleep(0) was a leftover
that was forgotten when porting to desktop Window's approach of handling
threads.

Change-Id: I5e3d6fb3eefe07407b910cc6a6b45781d320e151
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/thread/qthread_win.cpp

@@ -465,11 +465,7 @@ int QThread::idealThreadCount() Q_DECL_NOTHROW
 
 void QThread::yieldCurrentThread()
 {
-#if !defined(Q_OS_WINRT)
     SwitchToThread();
-#else
-    ::Sleep(0);
-#endif
 }
 
 #endif // QT_CONFIG(thread)