Volume: remove a useless `_delete_volume_type` alias
Volume: remove a useless `_delete_volume_type` alias

We shouldn"t allow people to alias client methods like this. It gives
a bad opinion on Tempest code base: lack of consistency, code harder
to read (because you constently have to switch between test code and
methods definition).

Change-Id: Ic52159ad72246cc5d8fc5ef3bb7877b8b8d8821b

openstack/tempest,bigswitch/tempest,cisco-openstack/tempest,masayukig/tempest,Tesora/tesora-tempest,masayukig/tempest,cisco-openstack/tempest,sebrandon1/tempest,vedujoshi/tempest,openstack/tempest,vedujoshi/tempest,sebrandon1/tempest,bigswitch/tempest,Juniper/tempest,Juniper/tempest,Tesora/tesora-tempest