Remove dead code from SystemServer

Remove dead code from SystemServer that obstensively protects devices
from negative (< 1970) system clock times, but does not.

In reality, SystemServer calls SystemClock.setCurrentTimeMillis(), which
calls through to the AlarmManager.setTime() binder call. When the code
runs the system server hasn't started any services yet, so this would
always fail.

AlarmManager has similar protections in place that do work: if the
device is earlier than a "build time", then the device's clock is forced
forward. That provides the protection that the code being removed from
SystemServer was trying to achieve.

Test: Added logging to SystemClock to prove AlarmManager is null /
manual debugging

Change-Id: I87dca1062829bc1a10202804fa7ee31e0a6f03a3