image: remove unused storage_repository attribute

Change-Id: I2a497a6b24ced14f20361db0c75a12d7085fb0bf
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27392
Reviewed-by: Dan Kenigsberg <danken@redhat.com>