api: remove the uploadVolume command

The uploadVolume command was never used by the engine and has been
obsoleted with the introduction of the imageSharing module.

Change-Id: Ie2b26aded19f482751196ef642386dcf92d81ce4
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21425
Reviewed-by: Sergey Gotliv <sgotliv@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>