code-owners should have been enabled for most hosts that depot_tools
supports by now. Remove our own implementation and rely on code-owners.

Change-Id: Iaf0d3db65b2e5063b67d42b92188c4ec51d2cd9a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3783475Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>
Reviewed-by: Joanna Wang <jojwang@chromium.org>
Commit-Queue: Gavin Mak <gavinmak@google.com>