selftests/bpf: Remove obsolete pahole/BTF support detection

Given lots of selftests won't work without recent enough Clang/LLVM that
fully supports BTF, there is no point in maintaining outdated BTF
support detection and fall-back to pahole logic. Just assume we have
everything we need.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20191011220146.3798961-3-andriin@fb.com