Remove the unused _api_version variable from volume tests

this variable was used to skip the v2 or v3 volume API test
based on below config options
    * ``CONF.volume-feature-enabled.api_v2``
    * ``CONF.volume-feature-enabled.api_v3``

Now those config options were deprecated and are going
to be removed from Tempest.

Volume tests run has been default to v3 and can be run against
v2 API via catalog_type config options. Any microversion
tests which cannot be run against v2 APIs will be skipped
by separate check of check_skip_with_microversion.

Change-Id: I6e05fbe21b29efa128fb59d16bf521d551b00304
10 files changed