Remove special handling of internal project for allowing maintainers to access merge request branches

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR should fix #211714 (closed).

As stated in the discussion there, the issue was introduced by 30cbf7c3, where checking for "allowing collaboration" was restricted for internal projects. This restriction solved the targeted issue (i.e. infinite loops). But, as a side effect, actually disabled collaboration with internal projects

In the meantime, a second and more targeted fix for the same issue had been supplied with ef4a03f8. This MR had introduced a special flag to "skip checking for allowed collaboration".

The one issue now has two fixes, meaning, we should be able to undo the old change and thus fix the new issue it had introduced. That's what I have done here.

Pipelines indicate, the code should not have unintended side-effects. I had to remove one check. But to my judgemend, this test actually only checked the specific way the first fix was implemented. So, this check seemed useless to me, once the old fix was undone.

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How to set up and validate locally

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see #211714 (closed). The branch should be rebase-able with this fix.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR as far a I could.

  However: I am a first-time contributor to this project and I have no means to test this change locally. I appreciate anyone willing to take that on! Also, this is my first time mangling with this project and this programming language.