Remove memory monitoring from the system watchdog

This was originally written as an in-case-we-need-it facility, but was
never actually used in production.  It also soaked up a surprising amount
of cpu on occasion, as well as doing sketchy things like demoting the
system_server's primary looper thread to the background cgroup at times.

Change-Id: I9a81a8d1e9caea9e0a1277d97785fe96add438d7