xprtrdma: Remove rpcrdma_buffer::rb_pool

Since commit 1e465fd4 ("xprtrdma: Replace send and receive
arrays"), this field is no longer used.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

net/sunrpc/xprtrdma/xprt_rdma.h

@@ -391,7 +391,6 @@ struct rpcrdma_buffer {
 	spinlock_t		rb_mwlock;	/* protect rb_mws list */
 	struct list_head	rb_mws;
 	struct list_head	rb_all;
-	char			*rb_pool;
 
 	spinlock_t		rb_lock;	/* protect buf lists */
 	int			rb_send_count, rb_recv_count;