Use Tempest-lib's compute flavors_client

Compute flavors_client has been migrated to tempest-lib-
I3de529b1e0db298530e54fd2c871d40f29614126

Now Tempest-lib provides this client as stable interface so Tempest
should start using this from lib and remove its own copy.
Also remove other files for this client like response schema, unit tests.

This commits makes Tempest to use compute flavors_client
from Tempest-lib.

Change-Id: I8e5b227bb1bf359c4a194f649dad186eb0574a46
7 files changed