Remove forkchoice dependency from stategen

[syjn99]

What type of PR is this?

Other

What does this PR do? Why is it needed?

1. State struct inside stategen package holds fc (ForkChoicer) but it is actually a dead field.

• Give forkchoice to stategen #11439
• It seems like we were trying to optimize with this field, but there's no work after Give forkchoice to stategen #11439.

2. fc is only useful as an argument to register a handler, which can also be done outside of stategen.New(..).

• Add a justified balance getter to forkchoice #11513
• Aditya's PR 18 #11945

With stategen.New(..) has fc in its function signature, we need to import doubly-linked-tree package in every test functions, mostly putting an empty forkchoice tree.

This PR removes the dead field (fc) and moves the handler registration in startStateGen.

Which issue(s) does this PR fix?

N/A

Other notes for review

Acknowledgements

• I have read CONTRIBUTING.md.
• I have included a uniquely named changelog fragment file.
• I have added a description with sufficient context for reviewers to understand this PR.
• I have tested that my changes work as expected and I added a testing plan to the PR description (if applicable).