xsk: Get rid of unused entry in struct xdp_buff_xsk

Get rid of the unused entry "unaligned" in struct xdp_buff_xsk.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210922075613.12186-2-magnus.karlsson@gmail.com