tst_QMetaType: remove call to pthread_yield()

It was Linux-only and now even Linux is complaining:

tst_qmetatype.cpp:421:26: warning: ‘int pthread_yield()’ is deprecated: pthread_yield is deprecated, use sched_yield instead [-Wdeprecated-declarations]

Change-Id: I0e5f6bec596a4a78bd3bfffd16cb1eadfa301f16
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

tests/auto/corelib/kernel/qmetatype/tst_qmetatype.cpp

@@ -417,9 +417,6 @@ protected:
             const QByteArray name = "Bar" + QByteArray::number(i) + postFix;
             const char *nm = name.constData();
             int tp = qRegisterMetaType<Bar>(nm);
-#if defined(Q_OS_LINUX) && !defined(Q_OS_ANDROID)
-            pthread_yield();
-#endif
             QMetaType info(tp);
             if (!info.isValid()) {
                 ++failureCount;