Removed 4.x deprecated code in .theme file

I've removed the code marked in the 4.x .theme file as deprecated, this work does also depend on the logo block rework happening in: https://www.drupal.org/project/prototype/issues/3366844 📌 Support Drupal branding block Fixed

Hi I rebased @jldust can you revise it ? please

Thank you for helping out, this has been merged and will be included in the next tagged release.