Remove engedy@ and msramek@ from UKM owners

...and clarify how the process works. One should get privacy review
on their proposal first, and then code review for the actual
implementation.

The privacy review should not be done on the CL, and the privacy team
can't comment on the technical aspects of the implementation.

Bug: None
Change-Id: Ic5f0c848b9cf2a89c04e2e3e05775bbcb8d624be
Reviewed-on: https://chromium-review.googlesource.com/793731Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Balazs Engedy <engedy@chromium.org>
Commit-Queue: Martin Šrámek <msramek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519711}

tools/metrics/ukm/PRIVACY_OWNERS

-# UKM metrics require additional considerations from the privacy side, so we use
-# a specific set of reviewers from metrics and privacy team.
+# UKM metrics require additional privacy considerations, so they use
+# a narrower set of reviewers than the rest of metrics codebase.
+#
+# Please make sure that your UKM collection proposal is approved by
+# the privacy team before sending out your CL for review.
+
 asvitkine@chromium.org
 bmcquade@chromium.org
-engedy@chromium.org
 holte@chromium.org
-msramek@chromium.org
 rkaplow@chromium.org