Remove github CODEOWNERS (apollographql#2885)

Github `CODEOWNERS` functionality sounds great in theory, but
the extra noise generated by constantly being assigned for review
on PR's is not necessary (we know PR's need to be looked at). We
also can't expect community members to always submit `Draft` PR's
when their changes are WIP, to help avoid code owner review
assignment. Maybe we'll re-enable this in the future, but for
now we're dropping it.