xsk: Remove unused inline function xsk_buff_discard()

commit f2f16758 ("xsk: Remove unused xsk_buff_discard")
left behind this, remove it.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Link: https://lore.kernel.org/bpf/20230616062800.30780-1-yuehaibing@huawei.com