Remove a misleading comment in PERMISSIONS_OWNERS

The comment implies that a review from one of these owners is required,
however owners of the directories in which permission_context files live
can also approve changes.

Change-Id: I94c7c8eb48340bd04214d02f42d5482805f0229f
Reviewed-on: https://chromium-review.googlesource.com/1059978Reviewed-by: Timothy Loh <timloh@chromium.org>
Commit-Queue: Raymes Khoury <raymes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558912}

chrome/browser/permissions/PERMISSIONS_OWNERS

-# PermissionContext subclasses require a Security UX review by one of the
-# following:
-
 benwells@chromium.org
 dominickn@chromium.org
 raymes@chromium.org