Remove dead code in system server initialization.

System server always forks from Zygote so we no longer need
the system_server executable which was probably broken anyhow.
This makes the initialization sequence slightly more intelligible.

Likewise, we don't need the GrimReaper anymore because init
will automatically take care of restarting the system when the
service manager dies.

Change-Id: I02c88d9392f7c8133d9cde9d0d978da89ed80452