udp: seq_file: Remove bpf_seq_afinfo from udp_iter_state

udp: seq_file: Remove bpf_seq_afinfo from udp_iter_state

This is a preparatory commit to remove the field. The field was
previously shared between proc fs and BPF UDP socket iterators. As the
follow-up commits will decouple the implementation for the iterators,
remove the field. As for BPF socket iterator, filtering of sockets is
exepected to be done in BPF programs.

Suggested-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Aditi Ghag <aditi.ghag@isovalent.com>
Link: https://lore.kernel.org/r/20230519225157.760788-5-aditi.ghag@isovalent.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>