[1.0.0-alpha1] Remove code specific to contrib themes

Also, remove this sentence from README.md:

If testing display_builder_page_layout, first set the theme before enable to allow a fixture to be loaded.

Would it be possible to keep display_builder_devel? I think it can become quite useful to debug display builder in a buggy state, and/or prevent deadlock.

Would it be possible to keep display_builder_devel?

I remove the sentence suggesting its removal. It is out of scope of this issue anyway.

modules/display_builder_entity_view/fixtures/default_entity_view.yml is empty.
Should be removed ?

Simply removing makes no sense! we need a solution to keep it and be able to use it. Created 📌 Fixtures Active