Revert "Add registry functions to instantiate models by provider (#428)"

[svlandeg]

This reverts commit ff07682.

Description

Moving #428 to the develop branch so we can first cut a 0.7.2 bugfix release without this big refactor.

Types of change

maintenance

Checklist

• I confirm that I have the right to submit this contribution under the project's MIT license.
• I ran all tests in tests and usage_examples/tests, and all new and existing tests passed. This includes
  • all external tests (i. e. pytest ran with --external)
  • all tests requiring a GPU (i. e. pytest ran with --gpu)
• My changes don't require a change to the documentation, or if they do, I've added all required information.