wallet, refactor: Remove Legacy wallet unused warnings and errors

[pablomartin4btc]

Remove dead code due to legacy wallet support removal.

[DrahtBot]

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Code Coverage & Benchmarks

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

Reviews

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #bitcoin-core/gui/872 (Menu action to export a watchonly wallet by achow101)
• #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.}

Hints

Try to run the tests locally, according to the documentation. However, a CI failure may still
happen due to a number of reasons, for example:

• Possibly due to a silent merge conflict (the changes in this pull request being
  incompatible with the current code in the target branch). If so, make sure to rebase on the latest
  commit of the target branch.

• A sanitizer issue, which can only be found by compiling with the sanitizer and running the
  affected test.

• An intermittent issue.

Leave a comment here, if you need help tracking down a confusing failure.

[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.