metrics library: complete switchover from /var/log to /var/run

This removes the double metrics even file use to facilitate
the transition.  The corresponding change has been in Chromium
for 2 days now.

BUG=chromium:373833
TEST=manually verified

Change-Id: I36ff355e6519b3894c0a4579758ae87bdac6b148
Reviewed-on: https://chromium-review.googlesource.com/200426


Reviewed-by: Bertrand Simonnet <bsimonnet@chromium.org>
Commit-Queue: Luigi Semenzato <semenzato@chromium.org>
Tested-by: Luigi Semenzato <semenzato@chromium.org>