fuzz: remove the `splicing` cfg gate from chanmon_consistency

[joostjager]

In PR #4561, splicing was disabled in fuzz testing because it was not ready yet and obscuring other issues. This is a tracking issue for re-enabling it again.

[alexgrad42]

Can I take this one as my first contribution to the repo? I've already tested the changes with the fuzzer and saw a good coverage increase

[joostjager]

I think there are still lingering bugs being worked on in splicing fuzzing. This is issue is just a reminder to remove the cfg flag afterwards.

[alexgrad42]

I've prepared this draft PR as a follow-up to #4581. I understand it might be too early to merge, but I'm opening this to show the verified coverage increase. Feel free to merge or request changes once the dependencies are ready.