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