meta: remove CODEOWNERS

The CODEOWNERS file is not working out. It is not pinging any groups
other than TSC (and more-or-less can't), and it pings TSC mostly for all
the wrong things.

It was a valiant effort, and possible worth revisiting at a future date
when functionality is closer to what we need. But for now, let us bid it
adieu.

PR-URL: #21161
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Ujjwal Sharma <usharma1998@gmail.com>