This removes the last direct access to the `utxos` table; all access
now goes through the Rust FFI.
`SDKSynchronizer.latestUTXOs` is removed without replacement. It was
introduced during the initial addition of shielding support, but:
- It is no longer used anywhere inside the SDK (when added, it was
used in a few other methods).
- It is not exposed in the `Synchronizer` protocol.
- It is AFAICT unused in Zashi iOS, Edge, and Unstoppable.
- It was functionally replaced by `refreshUTXOs`, which performs
best-effort UTXO updates instead of failing on any error. (It also
does not clear the `utxo`s table which makes it not equivalent.)
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