block/rnbd-srv: remove unused header

No need to include it since none of macros in limits.h are
used by rnbd-srv.

Acked-by: Jack Wang <jinpu.wang@ionos.com>
Signed-off-by: Guoqing Jiang <guoqing.jiang@linux.dev>
Link: https://lore.kernel.org/r/20230524070026.2932-3-guoqing.jiang@linux.dev


Signed-off-by: Jens Axboe <axboe@kernel.dk>