cros: metrics: network: Drop a few shill metrics

TimeOnline is broken since M104. TimeToDrop has been expired for a long
time and seems that no one is using that.

The platform code for emitting these metrics has been removed in
https://crrev.com/c/5401295 .

Bug: b/331562373
Test: None
Change-Id: I2aa13e9769d9713909b1d7539404c03063905174
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5404455


Reviewed-by: Matthew Wang <matthewmwang@chromium.org>
Commit-Queue: Jie Jiang <jiejiang@chromium.org>
Reviewed-by: Hugo Benichi <hugobenichi@google.com>
Cr-Commit-Position: refs/heads/main@{#1280117}