Revert "selftests: bpf: Don't try to read files without read permission"

This reverts commit 5bc60de5 ("selftests: bpf: Don't try to read
files without read permission").

Quoted commit does not work at all, and was never tested.
Script requires root permissions (and tests for them)
and os.access() will always return true for root.

The correct fix is needed in the bpf tree, so let's just
revert and save ourselves the merge conflict.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Cc: Jiri Pirko <jiri@resnulli.us>
Link: https://lore.kernel.org/bpf/20191101005127.1355-1-jakub.kicinski@netronome.com