xen: remove stale xs_input_avail() from header

In drivers/xen/xenbus/xenbus_comms.h there is a stale declaration of
xs_input_avail(). Remove it.
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>

drivers/xen/xenbus/xenbus_comms.h

@@ -42,7 +42,6 @@ int xb_write(const void *data, unsigned len);
 int xb_read(void *data, unsigned len);
 int xb_data_to_read(void);
 int xb_wait_for_data_to_read(void);
-int xs_input_avail(void);
 extern struct xenstore_domain_interface *xen_store_interface;
 extern int xen_store_evtchn;
 extern enum xenstore_init xen_store_domain_type;