Drop the custom `wp_notify_postauthor` function

We don't need it anymore and can just hook in to [`comment_notification_recipients`](https://developer.wordpress.org/reference/hooks/comment_notification_recipients/) instead to notify all coauthors of the new comment.

Related #346

Start by locating the custom wp_notify_postauthor function and its callers, then read the linked comment_notification_recipients hook documentation. Confirm how coauthors are currently notified and identify the relevant tests or notification behavior to verify. Done means the custom function is removed and all coauthors still receive notifications for new comments.

Written by the indexing model from the issue text.