selftests/bpf: drop unnecessary bpf_iter.h type duplication

selftests/bpf: drop unnecessary bpf_iter.h type duplication

Drop bpf_iter.h header which uses vmlinux.h but re-defines a bunch of
iterator structures and some of BPF constants for use in BPF iterator
selftests.

None of that is necessary when fresh vmlinux.h header is generated for
vmlinux image that matches latest selftests. So drop ugly hacks and have
a nice plain vmlinux.h usage everywhere.

We could do the same with all the kfunc __ksym redefinitions, but that
has dependency on very fresh pahole, so I'm not addressing that here.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20241029203919.1948941-1-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>