wallet: Replace SelectExternal with SetTxOut

Instead of having a separate CCoinControl::SelectExternal function, we
can use the normal CCoinControl::Select function and explicitly use
PreselectedInput::SetTxOut in the caller. The semantics of what an
external input is remains.

src/wallet/test/coinselector_tests.cpp

@@ -1282,7 +1282,7 @@ BOOST_AUTO_TEST_CASE(SelectCoins_effective_value_test)
     cc.m_allow_other_inputs = false;
     COutput output = available_coins.All().at(0);
     cc.SetInputWeight(output.outpoint, 148);
-    cc.SelectExternal(output.outpoint, output.txout);
+    cc.Select(output.outpoint).SetTxOut(output.txout);
 
     LOCK(wallet->cs_wallet);
     const auto preset_inputs = *Assert(FetchSelectedInputs(*wallet, cc, cs_params));

src/wallet/test/fuzz/coincontrol.cpp

@@ -60,7 +60,7 @@ FUZZ_TARGET(coincontrol, .init = initialize_coincontrol)
             },
             [&] {
                 const CTxOut tx_out{ConsumeMoney(fuzzed_data_provider), ConsumeScript(fuzzed_data_provider)};
-                (void)coin_control.SelectExternal(out_point, tx_out);
+                (void)coin_control.Select(out_point).SetTxOut(tx_out);
             },
             [&] {
                 (void)coin_control.UnSelect(out_point);