Its return type is change from `Approver` to `User`,
as code owner is type `User`, and it does not make sense
to wrap them in `Approver`.

When approvers are overriden, code owner is not included,
because the persisted approvers will include code owners already.

Add endpoint to delete approver by user_id,
since overall_approvers now returns User,
and this avoids one query for fetching user's approver association.