selftests/seccomp: Remove SIGSTOP si_pid check

Commit f149b315

 ("signal: Never allocate siginfo for SIGKILL or SIGSTOP")
means that the seccomp selftest cannot check si_pid under SIGSTOP anymore.
Since it's believed[1] there are no other userspace things depending on the
old behavior, this removes the behavioral check in the selftest, since it's
more a "extra" sanity check (which turns out, maybe, not to have been
useful to test).

[1] https://lkml.kernel.org/r/CAGXu5jJaZAOzP1qFz66tYrtbuywqb+UN2SOA1VLHpCCOiYvYeg@mail.gmail.com

Reported-by: Tycho Andersen <tycho@tycho.ws>
Suggested-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <shuah@kernel.org>