QmlDesigner: Remove duplicate assert

This assert was redundant, leading to duplicate warnings if triggered.

Change-Id: I59b6a7c17345a2ccba0d51ef50a303eec2026422
Reviewed-by: Mahmoud Badri <mahmoud.badri@qt.io>

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

@@ -873,7 +873,8 @@ void AbstractView::assignMaterialTo3dModel(const ModelNode &modelNode, const Mod
 
     ModelNode matLib = materialLibraryNode();
 
-    QTC_ASSERT(matLib.isValid(), return);
+    if (!matLib.isValid())
+        return;
 
     ModelNode newMaterialNode;