style: remove redundant instance->owned = true (#2723)

which was just before set to True in instance->allocate_layout()

include/pybind11/detail/class.h

@@ -340,8 +340,6 @@ inline PyObject *make_new_instance(PyTypeObject *type) {
     // Allocate the value/holder internals:
     inst->allocate_layout();
 
-    inst->owned = true;
-
     return self;
 }