If code-owners plugin is enable for the repo, skip owners check on
commit, and skip checking owners format, as that will be done by
the plugin.

Change-Id: I1663baef4f0f27b00423071343fe740f6da50ce7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2727131
Auto-Submit: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>