block/rnbd: kill rnbd_flags_supported

This routine is not called since added. Then the two flags
(RNBD_OP_LAST and RNBD_F_ALL) can be removed too after kill
rnbd_flags_supported.

Acked-by: Jack Wang <jinpu.wang@ionos.com>
Signed-off-by: Guoqing Jiang <guoqing.jiang@linux.dev>
Link: https://lore.kernel.org/r/20230524070026.2932-2-guoqing.jiang@linux.dev


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