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: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>