iOS: fix assert when using QOffscreenSurface

Qt defaults to creating a QWindow as offscreen surface
if QPlatformIntegration::createPlatformOffscreenSurface
returns 0. Using an offscreen surface is often done
in a background thread, which is problematic, since then
a QIOSWindow will also be created in a background thread.
According to Apple docs, working with a UIView from other
threads than the main thread should not be done. In our
case, we instead hit an assert in QApplication that
checks for the same.

As a quick fix for Qt 5.2, we remove the offending call that
causes the assert, since we anyway will call the same function
lazily when becoming first responder.

Task-number: QTBUG-35378

Change-Id: Id35462f99783a9748c688b163f6497de9bfff73e
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>

src/plugins/platforms/ios/qioswindow.mm

@@ -108,8 +108,6 @@
             [NSNumber numberWithBool:YES], kEAGLDrawablePropertyRetainedBacking,
             kEAGLColorFormatRGBA8, kEAGLDrawablePropertyColorFormat, nil];
 
-        [self updateTextInputTraits];
-
         if (isQtApplication())
             self.hidden = YES;