Do not clear materials when setting mesh

(cherry picked from commit 6e2a55c4641a8b9a456a64694077f9428da8d200)

scene/3d/mesh_instance.cpp

@@ -115,7 +115,6 @@ void MeshInstance::set_mesh(const Ref<Mesh> &p_mesh) {
 
 	if (mesh.is_valid()) {
 		mesh->disconnect(CoreStringNames::get_singleton()->changed, this, SceneStringNames::get_singleton()->_mesh_changed);
-		materials.clear();
 	}
 
 	if (skin_ref.is_valid() && mesh.is_valid() && _is_software_skinning_enabled() && is_visible_in_tree()) {