Remove redundant check that causes lvm cache to refresh every volume creation

We try to create a volume without validating if it exists first.
If the volume exists we need to make sure we don't it delete in
rollback.

Change-Id: Ib6f3b6ca8313070d0345b5f76ebb0b3d9772d14f
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=979193
Signed-off-by: Yeela Kaplan <ykaplan@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/18274
Reviewed-by: Ayal Baron <abaron@redhat.com>