iOS: Remove forced Qt::AA_ShareOpenGLContexts

Commit 8e70241dcc removed the need for having a global sharecontext
to be able to composit GL and raster content in the backingstore, but
forgot to remove this part of the iOS platform.

Change-Id: I46ffd766729369aff2f8c54799bd033905c9f3dc
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

src/plugins/platforms/ios/qiosintegration.mm

@@ -94,10 +94,6 @@ QIOSIntegration::QIOSIntegration()
                "'applicationDidFinishLaunching' inside your UIApplication delegate.\n");
     }
 
-    // The backingstore needs a global share context in order to support composition in
-    // QPlatformBackingStore.
-    qApp->setAttribute(Qt::AA_ShareOpenGLContexts, true);
-
     // Set current directory to app bundle folder
     QDir::setCurrent(QString::fromUtf8([[[NSBundle mainBundle] bundlePath] UTF8String]));