This change removes leftover unused code from the HTML editor that should have been deleted in an earlier cleanup. It has no expected impact on users, but helps keep the editor codebase simpler and easier to maintain.

`split_unsplittable_handlers` was not removed in [1] by mistake.

[1]: https://github.com/odoo/odoo/commit/75f6f0a20cb7a860a703e73e80a4247e716ab620