svcrdma: Remove stale comment for svc_rdma_wc_receive()

xprt pinning was removed in commit 365e9992 ("svcrdma: Remove
transport reference counting"), but this comment was not updated
to reflect that change.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

net/sunrpc/xprtrdma/svc_rdma_recvfrom.c

@@ -322,8 +322,6 @@ bool svc_rdma_post_recvs(struct svcxprt_rdma *rdma)
  * @cq: Completion Queue context
  * @wc: Work Completion object
  *
- * NB: The svc_xprt/svcxprt_rdma is pinned whenever it's possible that
- * the Receive completion handler could be running.
  */
 static void svc_rdma_wc_receive(struct ib_cq *cq, struct ib_wc *wc)
 {