Remove the Panelizer support

Describe your bug or feature request.

The Panelizer project page has the following note:

With the Layout Builder system committed, much of Panelizer's functionality is now available out of the box in Drupal core.

Therefore, Panelizer 5.x is solely a migration path to Layout Builder. If you update from Panelizer 4.x to 5.x, your Panelizer layouts will continue to work as before, but you will not be able to add or edit default layouts! You can still change custom layouts for individual entities, but the UI for adding or editing default layouts has been removed and replaced with a big, friendly button that says "Migrate to Layout Builder".

Also note that Panelizer 5.x is still in alpha, and the migration path is NOT yet ready for prime time. That means it may result in data loss or other weirdness. You should NOT use Panelizer 5.x in production at this time.

There is no reason for us to keep supporting Panelizer in Commerce 3.x, and Panelizer is one of the only remaining blockers for running tests properly with D11 (due to the require-dev dependency).

Let's remove the dependency and the Panelizer entity integration for products.