fuzz: remove unsupported coins cache cursor fuzz probe #35562

Problem: The coins_view fuzz target checked that CCoinsViewCache::Cursor() throws, but that method is intentionally unsupported and always throws: https://github.com/bitcoin/bitcoin/blob/93c6ea147422199777ddbc5f6d04134b82420b76/src/coins.h#L450-L452

Fix: Remove the input-independent throw check from the fuzz target. The target still exercises the useful cache/view behavior, and keeps DB-backed cursor coverage where cursor iteration is supported. Some leftover scope-guards were also removed.

This was extracted from review discussion in #35295 (review).

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35562.

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Reviews

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This is cheap check, do we really need to remove this?

do we really need to remove this?

Isn't it just fake coverage? Why would we fuzz test a deterministic throw?

I think it is fine to pin behaviour here. We could do the same with a unit test I guess, but not sure if that is worth a pull request. Do you remember why this was an error and not just a hard assert?

EDIT: Was curious why this is even a virtual method, and seems like it doesn't need to be. The few users can get it from the CCoinsViewDB directly.