Remove unused dm_flush_fn .flush target method from header.

This was left-over from the FLUSH/FUA conversion and is no longer used.
Signed-off-by: NJoe Thornber <ejt@redhat.com>
Signed-off-by: NMike Snitzer <snitzer@redhat.com>
Signed-off-by: NAlasdair G Kergon <agk@redhat.com>