remove a useless volatile

... introduced with 8c113ff6

Here it seems the intention was to make sure that the access to the
parameter happens uncached, but even then it would not have helped.

bin/varnishd/cache/cache_ban_lurker.c

@@ -408,7 +408,7 @@ void * v_matchproto_(bgthread_t)
 ban_lurker(struct worker *wrk, void *priv)
 {
 	struct vsl_log vsl;
-	volatile vtim_real d;
+	vtim_real d;
 	vtim_dur dt;
 	unsigned gen = ban_generation + 1;