Do not assume vol.encryption is enabled when barbican is on

The two conditions are unrelated. Barbican may be around
for example to support glance, but the cinder backend in use
may not support encryption (currently any NFS-based cinder backend).

Change-Id: I11cbea19b910df3c9dbac3178940304c1135cedb

config_tempest/services/compute.py

@@ -58,10 +58,6 @@ class ComputeService(VersionedService):
             C.LOG.info('Can not retrieve hosts, user are not allowed')
             return 1
 
-    def post_configuration(self, conf, is_service):
-        conf.set('compute-feature-enabled', 'attach_encrypted_volume',
-                 str(is_service(**{'type': 'key-manager'})))
-
     @staticmethod
     def get_service_type():
         return ['compute']