task: remove unused _addTask method

Change-Id: I1f07ebbde4d59b0ca02896063aeeee3c998cbb95
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26770
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>