UPSTREAM: seccomp: remove 2-phase API

Since nothing is using the 2-phase API, and it adds more complexity than
benefit, remove it.

Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Andy Lutomirski <luto@kernel.org>
(cherry picked from commit 8112c4f140fa03f9ee68aad2cc79afa7df5418d3)

Bug: 119769499
Change-Id: Iff6246c1e6e9dd0161b80b666a5e796f78a5c785
Signed-off-by: Greg Hackmann <ghackmann@google.com>