OS-531 Do not return None in infoblox_db

As per [review comments](https://review.openstack.org/#/c/132313/1/neutron/db/infoblox/infoblox_db.py) from garyk.

python returns None by default, remove unnecessary code from
infoblox_db.

Change-Id: Id72c6c8798a7d3e41a5c2e2ef5a65ad462bf0c9a