metrics: remove unused constant

A constant was left during the refactoring
(https://chromium-review.googlesource.com/#/c/207237/) and it is failing the
generic ASAN buildbot.

BUG=chromium:374368
TEST=FEATURES=test emerge-amd64-generic metrics

Change-Id: Ief9f2a064dbeded87e2e5fc67d3659fedac88867
Reviewed-on: https://chromium-review.googlesource.com/207460


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