Cinder consistency group has been replaced by the generic group feature.
Horizon support of the generic group (in the project dashboard) is
available since Rocky release and it covers all existing support
for consistency group in horizon.
The consistency group support is horizon was marked as deprecated
in Stein release [1].
This commit drops the consistency group support.
[1] https://review.openstack.org/#/c/626846/
Change-Id: I11187d2b03b7e0033a6c6ba3f8be25b8b5e4dd74
431fd6c16b