Remove XML support from flavor v3 API plugins

Remove XML support from the following v3 compute API plugins:

 - flavor_access
 - flavor_manage
 - flavor_rxtx
 - flavors
 - flavors_extraspecs

Part of blueprint remove-v3-xml-api

DocImpact

Change-Id: I9745285cb6053dcb634c23002f1f24412dc62021
Merge "Remove XML support from flavor v3 API plugins"