seccomp-util: include @sandbox in @default

seccomp-util: include @sandbox in @default

Every services and containers should be able to protect their users and
limit the impact of security bugs thanks to the security syscalls
provided by seccomp and Landlock.  The goal of these syscalls is to
improve security with additional restrictions.  They are designed to be
safely used by unprivileged (and then potentially malicious) users.

Remove the now-redundant "seccomp" entry for nspawn.