bpf: Don't explicitly emit BTF for struct btf_iter_num

Commit 6018e1f4 ("bpf: implement numbers iterator") added the
BTF_TYPE_EMIT line that this patch is modifying. The struct btf_iter_num
doesn't exist, so only a forward declaration is emitted in BTF:

  FWD 'btf_iter_num' fwd_kind=struct

That commit was probably hoping to ensure that struct bpf_iter_num is
emitted in vmlinux BTF. A previous version of this patch changed the
line to emit the correct type, but Yonghong confirmed that it would
definitely be emitted regardless in [0], so this patch simply removes
the line.

This isn't marked "Fixes" because the extraneous btf_iter_num FWD wasn't
causing any issues that I noticed, aside from mild confusion when I
looked through the code.

  [0]: https://lore.kernel.org/bpf/25d08207-43e6-36a8-5e0f-47a913d4cda5@linux.dev/Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20231013204426.1074286-2-davemarchevsky@fb.com

kernel/bpf/bpf_iter.c

@@ -793,8 +793,6 @@ __bpf_kfunc int bpf_iter_num_new(struct bpf_iter_num *it, int start, int end)
 	BUILD_BUG_ON(sizeof(struct bpf_iter_num_kern) != sizeof(struct bpf_iter_num));
 	BUILD_BUG_ON(__alignof__(struct bpf_iter_num_kern) != __alignof__(struct bpf_iter_num));
 
-	BTF_TYPE_EMIT(struct btf_iter_num);
-
 	/* start == end is legit, it's an empty range and we'll just get NULL
 	 * on first (and any subsequent) bpf_iter_num_next() call
 	 */