xen/blkback: remove stale prototype

There is no function xen_blkif_purge_persistent(), so remove its
prototype from common.h.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Juergen Gross <jgross@suse.com>