open(2): Remove O_BENEATH and AT_BENEATH
with the reasoning that the flags did not worked properly, and were not
shipped in a release.
O_RESOLVE_BENEATH is kept as useful.
Reviewed by: markj
Tested by: arichardson, pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D28907