mkspec (iOS): use QTPLUGIN.platforms=-

After bbcdccd, a deprecation warning is written to
the console every time you run qmake on iOS.

This change will make use of the new QTPLUGIN.platforms=-
instead if the deprecated CONFIG -= import_qpa_plugin

Change-Id: I51e4f9d18f6abd87512a39b3236b89d5444fd6c1
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>

mkspecs/macx-ios-clang/features/qt.prf

@@ -15,7 +15,7 @@ equals(TEMPLATE, app):contains(QT, gui(-private)?) {
     # We do link and dependency resolution for the platform plugin
     # manually, since we know we always need the plugin, so we don't
     # need to generate an import for it.
-    CONFIG -= import_qpa_plugin
+    QTPLUGIN.platforms = -
 
     !no_main_wrapper {
         # We use ld to rename the _main symbol to _qt_main, so that we don't get a symbol clash