Remove instance_type* proxy methods from nova.db.api

There were several proxy methods left in nova.db.api to ease the rename
of instance_type to flavor with a note to remove them later.  This
patch does that removal and handles the impacts to the tests.

Part of blueprint flavor-instance-type-dedup

Change-Id: I2dd7a68ea028056996da6ea96a1fe3cae4ad0389
Merge "Remove instance_type* proxy methods from nova.db.api"