Remove orphaned db method instance_test_and_set

Commit 86544fad81f95b74407a76bea8b081f490e2832f removed the only use
of this method together with the sqlalchemy implementation of it but
forgot to remove it from db.api.

Change-Id: I751f99ee64cdeab56f1f3fd9f846157f241804a2
Merge "Remove orphaned db method instance_test_and_set"