Remove old navigation design from CHT

[latin-panda]

Description
Remove the old navigation code to simplify the maintenance of CHT.

CHT released the new navigation design in v4.12. Projects upgrading to 4.12 will automatically have the new navigation design. However, if necessary, the old navigation can be shown by enabling permissions for specific user roles (can_view_old_navigation). This capability/permission is considered deprecated and will be removed in a future release (communications about this deprecation have been done in round-up calls, release notes, and forum posts from the start of this project)

Having both adds extra complexity to the maintainers' efforts to add, modify, or fix features in CHT. They must ensure that each feature is displayed correctly and works well in new and old designs.

Six months after v4.12 release, it's recommended to evaluate if partners use can_view_old_navigation and schedule removing the old navigation accordingly.

[latin-panda]

@andrablaj @jkuester, please monitor this. It will improve the contributors' experience working with CHT.
While keeping both designs, ensure the contributors test their work in the old and new designs. I opened a PR to add a reminder for when someone opens a PR.

[jkuester]

Tagging this for the 5.0.0 release if we do not do it sooner.