macOS: Remove QMacRootLevelAutoReleasePool member from QCoreApplication

Tying the pool to the lifetime of  QCoreApplication becomes a problem when
the application is allocated on the heap, for example when Qt is used to
create plugins, as NSAutoreleasePool should be allocated and drained in
the same context, typically a function scope.

The feature is disabled for now, until we can find a better way to
achieve the same thing.

Change-Id: I600587df62a8cc7caded5172ad914c0e04d794d9
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/corelib/kernel/qcoreapplication_p.h

@@ -89,10 +89,6 @@ public:
     QString appName() const;
     QString appVersion() const;
 
-#ifdef Q_OS_MACOS
-    QMacRootLevelAutoReleasePool autoReleasePool;
-#endif
-
 #ifdef Q_OS_DARWIN
     static QString infoDictionaryStringProperty(const QString &propertyName);
 #endif