Remove renderCount from glTF mesh instancing test

tests/validation/config.json

@@ -831,7 +831,6 @@
         {
             "title": "GLTF Extension EXT_mesh_gpu_instancing",
             "playgroundId": "#QFIGLW#9",
-            "renderCount": 50,
             "referenceImage": "gltfExtensionExtMeshGpuInstancingTest.png"
         }
     ]