sd: remove unused volumeExists method

Last used by: http://gerrit.ovirt.org/18274

Change-Id: I3d95e281c1f9824ec690db40f1d5fb62404f11e0
Signed-off-by: Yeela Kaplan <ykaplan@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/36209
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>