Remove tnagel from authpolicy/OWNERS

I'm not working on this codebase anymore.

Bug: none
Change-Id: I8b677a6fe89cf0b53307fb787a3470d8408a3025
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2526101Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Commit-Queue: Roman Sorokin [CET] <rsorokin@chromium.org>
Commit-Queue: Thiemo Nagel <tnagel@chromium.org>
Auto-Submit: Thiemo Nagel <tnagel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825302}

chromeos/dbus/authpolicy/OWNERS

 rsorokin@chromium.org
-tnagel@chromium.org