Remove unused cert db method.

nova.db.sqlalchemy.api contained a certificate_get() method that was
unused.  It was not in the main db.api.  This method was also the only
reference to CertificateNotFound, so remove that as well.

Change-Id: Iba9aece2302e2f175e5d60e48b23959885a81c8d
Merge "Remove unused cert db method."