capsicum: remove LSM hook for intercept_syscall

Rather than having an LSM hook that Capsicum implements for
intercepting syscalls, instead call straight through to
Capsicum.
seccomp: remove in-tree selftests

A better place to add selftests for seccomp is in the
 https://github.com/redpig/seccomp
repository; this is the main test suite used by the
seccomp maintainers.