Delete NetworkStatsFactoryBenchmark

As commented in the test, it is broken today and barely builds.

Nothing seems to be referencing this library anywhere: remove it.

Ignore-AOSP-First: submitting in AOSP separately from topic
Bug: 197717846
Test: m
Change-Id: I0dd12dfd3f9bf99c9723d58e2dc656ea7db3a1f3