bpf: Remove unused macro

Commit 7aebfa1b ("bpf: Support narrow loads from bpf_sock_addr.user_port")
removed one and only SOCK_ADDR_LOAD_OR_STORE_NESTED_FIELD callsite but kept
the macro. Remove it to clean up the code base. Found while getting lost in
the BPF code.

Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20241001200605.249526-1-maciej.fijalkowski@intel.com