wallet: Remove COutput::spendable and AvailableCoinsListUnspent

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.

src/wallet/test/coinselection_tests.cpp

@@ -53,7 +53,7 @@ static OutputGroup MakeCoin(const CAmount& amount, bool is_eff_value = true, Coi
     tx.vout[0].nValue = amount + int(is_eff_value) * fees;
     tx.nLockTime = next_lock_time++;        // so all transactions get different hashes
     OutputGroup group(cs_params);
-    group.Insert(std::make_shared<COutput>(COutPoint(tx.GetHash(), 0), tx.vout.at(0), /*depth=*/1, /*input_bytes=*/custom_spending_vsize, /*spendable=*/true, /*solvable=*/true, /*safe=*/true, /*time=*/0, /*from_me=*/false, /*fees=*/fees), /*ancestors=*/0, /*descendants=*/0);
+    group.Insert(std::make_shared<COutput>(COutPoint(tx.GetHash(), 0), tx.vout.at(0), /*depth=*/1, /*input_bytes=*/custom_spending_vsize, /*solvable=*/true, /*safe=*/true, /*time=*/0, /*from_me=*/false, /*fees=*/fees), /*ancestors=*/0, /*descendants=*/0);
     return group;
 }