MAINTAINERS: Delete inactive maintainers from AF_XDP

Delete Björn Töpel and Jonathan Lemon as maintainer and reviewer,
respectively, as they have not been contributing towards AF_XDP for
several years. I have spoken to Björn and he is ok with his removal.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Acked-by: Björn Töpel <bjorn@kernel.org>
Link: https://lore.kernel.org/bpf/20250915120148.2922-1-magnus.karlsson@gmail.com