QmlDesigner: Fix test testRewriterSignalDefinition

Otherwise the check if the property was handled will throw an assert.

Change-Id: Ifc2fab22cd10ae811cd36137196c1ed6ba82677e
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Marco Bubke <marco.bubke@qt.io>

src/plugins/qmldesigner/designercore/model/texttomodelmerger.cpp

@@ -1372,6 +1372,7 @@ void TextToModelMerger::syncNode(ModelNode &modelNode,
                 }
 
                 syncSignalDeclarationProperty(modelProperty, signature, differenceHandler);
+                modelPropertyNames.remove(astName.toUtf8());
                 continue; // Done
             }