tools/bpf: Remove bpf-helpers from bpftool docs

This logic is used for validating the manual pages from selftests, so
move the infra under tools/testing/selftests/bpf/ and rely on selftests
for validation rather than tying it into the bpftool build.

Signed-off-by: Joe Stringer <joe@cilium.io>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/bpf/20210302171947.2268128-12-joe@cilium.io