Reduce unused code

[SimonDold]

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[maltehelmert]

Was this created by accident? You did not replace the placeholder with the required content.

Regarding the pull request, the forward declarations are not "unused code" but there intentionally. We thought it was clearer to forward-declare everything but think about what does or doesn't need forward declarations here. It also serves as an overview of the large collection of classes that work in a coordinated way. So this part is not something we should merge.

The "get_initial_state" looks like it should go (but I didn't check context thoroughly).