QuickDesigner: Fix compilation of QuickDesigner/core tests

Commit 112df597 removed the definition, but not declaration of
test method.

Reviewed-by: erikv

tests/auto/qml/qmldesigner/coretests/testcore.h

@@ -164,7 +164,6 @@ private slots:
     void testCopyModelRewriter1();
     void testCopyModelRewriter2();
     void testSubComponentManager();
-    void testComponentLoadingTabWidget();
     void testAnchorsAndRewriting();
     void testAnchorsAndRewritingCenter();