In descriptor wallets, we consider all outputs to be spendable as we no
longer have mixed mine and watchonly in a wallet. As such,
COutput::spendable is meaningless and can be removed.
Furthermore, CoinFilterParams::only_spendable can be removed as that was
essentially checking for COutput::spendable.
Lastly, AvailableCoinsListUnspent can also be removed as the wrapper is
now only setting the feerate to std::nullopt which is trivial enough that
a dedicated wrapper is not needed.
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