wallet, refactor: Remove Legacy wallet unused warnings and errors

[pablomartin4btc]

Remove dead code due to legacy wallet support removal.

These changes have no impact on functionality. They are transparent to the end user, as legacy wallets can't be created or loaded anymore, so these checks are no longer reached. The legacy-to-descriptor wallet migration flow is not affected either, as these removals are not part of its process.

[DrahtBot]

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/32481.

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ACK	rkrux, achow101

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #28333 (wallet: Construct ScriptPubKeyMans with all data rather than loaded progressively by achow101)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

[DrahtBot]

🚧 At least one of the CI tasks failed.
_{Task lint: https://github.com/bitcoin/bitcoin/runs/42142854745
LLM reason (✨ experimental): The CI failure is due to trailing whitespace and spelling errors identified by the lint check.}

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[pablomartin4btc]

Updates:

• Addressed @achow101's feedback by adding an Assert() on CreateWallet() enforcing only descriptor wallets can be created.

[pablomartin4btc]

Addressed feedback removing unnecessary descriptor wallet checks.

[pablomartin4btc]

Updates:

• Removed "non-descriptor" checks and error instances in RPC (noticed while reviewing rpc, doc: update listdescriptors RCP help #32708).

[rkrux]

Looking good based on a cursory glance, will take a deeper look soon.
Maybe mention a note in the PR description about wallet migration and how removing these checks don't affect that flow?

[pablomartin4btc]

Updates:

• Addressed feedback from @maflcko and @achow101.
• Removed also wallet descriptor check from CWallet::EncryptWallet() and CWallet::Create().

[rkrux]

utACK ce90f0c

Looks fine to me, I navigated through the codebase to verify the diff's correctness. Suggested one more dead code removal if makes sense.

[achow101]

ACK ce90f0c