lib/tempest: Liberty EOL: remove an useless feature flag
lib/tempest: Liberty EOL: remove an useless feature flag

The snapshot_backup feature flag was introduced in
Ib695e60c2ed7edf30c8baef9e00f0307b1156551 to enable Tempest tests
introduced in I1964ce6e1298041f8238d76fa4b7029d2d23bbfb

But I1964ce6e1298041f8238d76fa4b7029d2d23bbfb was never merged so that
feature flag was never really useful.

Change-Id: I4e0bc786d2320907cb101fc788ad51444628537d

nawawi/openstack,richliu/devstack,noironetworks/devstack,bswartz/devstack,richliu/devstack,group-policy/devstack,group-policy/devstack,openstack-dev/devstack,nawawi/openstack,bswartz/devstack,openstack-dev/devstack,dtroyer/devstack,noironetworks/devstack,dtroyer/devstack