selftests/bpf: Remove SEC("version") from test progs

Since commit 6c4fc209fc ("bpf: remove useless version check for prog
load") these "version" sections, which result in bpf_attr.kern_version
being set, have been unnecessary.

Remove them so that it's obvious to folks using selftests as a guide that
"modern" BPF progs don't need this section.

Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211007231234.2223081-1-davemarchevsky@fb.com

tools/testing/selftests/bpf/progs/test_stacktrace_build_id.c

@@ -73,4 +73,3 @@ int oncpu(struct random_urandom_args *args)
 }
 
 char _license[] SEC("license") = "GPL";
-__u32 _version SEC("version") = 1; /* ignored by tracepoints, required by libbpf.a */