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 <[email protected]>
Reviewed-By: Michael Dawson <[email protected]>
Reviewed-By: Ali Ijaz Sheikh <[email protected]>
Reviewed-By: Benjamin Gruenbaum <[email protected]>
Reviewed-By: Matteo Collina <[email protected]>
Reviewed-By: Tiancheng "Timothy" Gu <[email protected]>
Reviewed-By: Ujjwal Sharma <[email protected]>