design: simplify how luxid determins job termination

With the job files, we already have designated files per job. So
there is no need to additionally signal through a socket regular
job termination. This design also has the advantage that it is
already implemented and working in current luxid.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Petr Pudlak <pudlak@google.com>