Remove uses of instance['instance_type'] from nova/api

This removes a test that attempted to delete the data from the
instance['instance_type'] structure and confirm that the flavor can
still be fetched. Since this data is no longer consulted in that
action (and will be removed altogether soon), this test is removed.

This is one change in a series aimed at removing the use of instance-linked
instance_type objects, in favor of the decoupled type data in
system_metadata. See bug 1140119 for more details.

Change-Id: I3ba56f9419aece5b9269a181264d09b117ecdb81
Merge "Remove uses of instance['instance_type'] from nova/api"