Remove deprecated public Backbone and Underscore libraries

Problem/Motivation

In #3258931: Deprecate backbone and tag it internal and #3272872: deprecate underscore and mark it internal, we deprecated the Backbone and Underscore libraries as public API. There are separate internal libraries we are using as we decouple core's JS from Backbone, and we can't remove the dependencies themelves yet, but we can already remove at least the public libraries from D10.

Proposed resolution

Remove the deprecated libraries from 10.0.x. Leave the internal libraries and vendor assets in place until they, too, can be properly removed.

Remaining tasks

TBD

User interface changes

API changes

Data model changes

Release notes snippet

The Backbone and Underscore core JavaScript dependencies are no longer provided as public core libraries. Backbone and Underscore will eventually be removed from core, possibly prior to Drupal 10.0.0. Modules or themes which depend on these libraries should either refactor their code to remove the dependencies, or treat them as third-party dependencies for the contributed module.