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>

drivers/block/xen-blkback/common.h

@@ -384,7 +384,6 @@ void xen_blkif_xenbus_fini(void);
 
 irqreturn_t xen_blkif_be_int(int irq, void *dev_id);
 int xen_blkif_schedule(void *arg);
-int xen_blkif_purge_persistent(void *arg);
 void xen_blkbk_free_caches(struct xen_blkif_ring *ring);
 
 int xen_blkbk_flush_diskcache(struct xenbus_transaction xbt,