SUNRPC: Remove comment for sp_lock

It is obsolete since sp_lock was discarded in commit 580a2575
("SUNRPC: discard sp_lock").

Signed-off-by: Guoqing Jiang <guoqing.jiang@linux.dev>
Reviewed-by: NeilBrown <neilb@suse.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>