buf_ring: Remove PREFETCH_DEFINED

I'm not able to find anything in the tree that ever defined it. Remove
as it's unused so is untested.

Reviewed by:	alc, imp, kib, markj
Sponsored by:	Arm Ltd
Differential Revision:	https://reviews.freebsd.org/D46148

sys/sys/buf_ring.h

@@ -165,9 +165,6 @@ static __inline void *
 buf_ring_dequeue_sc(struct buf_ring *br)
 {
 	uint32_t cons_head, cons_next;
-#ifdef PREFETCH_DEFINED
-	uint32_t cons_next_next;
-#endif
 	uint32_t prod_tail;
 	void *buf;
 
@@ -204,20 +201,10 @@ buf_ring_dequeue_sc(struct buf_ring *br)
 	prod_tail = atomic_load_acq_32(&br->br_prod_tail);
 
 	cons_next = (cons_head + 1) & br->br_cons_mask;
-#ifdef PREFETCH_DEFINED
-	cons_next_next = (cons_head + 2) & br->br_cons_mask;
-#endif
 
 	if (cons_head == prod_tail) 
 		return (NULL);
 
-#ifdef PREFETCH_DEFINED	
-	if (cons_next != prod_tail) {		
-		prefetch(br->br_ring[cons_next]);
-		if (cons_next_next != prod_tail) 
-			prefetch(br->br_ring[cons_next_next]);
-	}
-#endif
 	br->br_cons_head = cons_next;
 	buf = br->br_ring[cons_head];