api: remove obsolete volume prepare and teardown

The prepare and teardown volume API were never officially supported,
engine never used them and they were never exposed in vdsClient.
Additionally as reported in rhbz#1104672 prepareVolume was broken.
In a recent change (de40063) we exposed prepareImage and teardownImage
providing a similar functionality.

Relates-To: https://bugzilla.redhat.com/show_bug.cgi?id=1104672
Change-Id: Ic101ebc6e62e83b06b268c52e146cf0740fab887
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28390
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>