No need to delete self._subnet_objs[vn_fq_name_str] since _create_subnet_objs will start with empty dict anyway. Deleting it can cause a race condition if multiple updates are received for the same network at the same time.

Change-Id: Ibef11ed139f0edcf3789c210ce23b4700e20f442
Closes-Bug: 1369797
Merge "No need to delete self._subnet_objs[vn_fq_name_str] since _create_subnet_objs will start with empty dict anyway. Deleting it can cause a race condition if multiple updates are received for the same network at the same time."