Fixes custom endpoint usage for OpenAI models

[rmitsch]

Description

Fixes custom endpoint usage for OpenAI models by removing a check for the endpoint to be one of the default endpoints.

Corresponding documentation PR

-

Types of change

Bug fix.

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.

[svlandeg]

LGTM!