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
1 file changed