tools/xenstored: Remove unused prototype

A prototype for dump_conn() has been present for quite a long time
but there are no implementation. Even, AFAICT in the patch that
introduced it. So drop it.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Reviewed-by: Juergen Gross <jgross@suse.com>

tools/xenstore/xenstored_core.c

@@ -2085,7 +2085,6 @@ static struct option options[] = {
 #endif
 	{ NULL, 0, NULL, 0 } };
 
-extern void dump_conn(struct connection *conn); 
 int dom0_domid = 0;
 int dom0_event = 0;
 int priv_domid = 0;