Add maruel, remove cmp&darin from //testing/buildbot/OWNERS.

As part of issues 496321 and 98637, I'll start sending a fair number of
(somewhat) mechanical changes.

cmp and darin are not coding on the chromium project anymore.

R=jam@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1157673005

Cr-Commit-Position: refs/heads/master@{#332993}
1 file changed