wallet: skip available coins fetch if "other inputs" are disallowed

no need to waste resources calculating the wallet available coins if
they are not going to be used.

The 'm_allow_other_inputs=true` default value change is to correct
an ugly misleading behavior:

The tx creation process was having a workaround patch to automatically
fall back to select coins from the wallet if `m_allow_other_inputs=false`
(previous default value) and no manual inputs were selected.

This could be seen in master in flows like `sendtoaddress`, `sendmany`
and even the GUI, where the `m_allow_other_inputs` value isn't customized
and the wallet still selects and adds coins to the tx internally.

src/qt/sendcoinsdialog.cpp

@@ -289,7 +289,9 @@ bool SendCoinsDialog::PrepareSendText(QString& question_string, QString& informa
 
     updateCoinControlState();
 
-    prepareStatus = model->prepareTransaction(*m_current_transaction, *m_coin_control);
+    CCoinControl coin_control = *m_coin_control;
+    coin_control.m_allow_other_inputs = !coin_control.HasSelected(); // future, could introduce a checkbox to customize this value.
+    prepareStatus = model->prepareTransaction(*m_current_transaction, coin_control);
 
     // process prepareStatus and on error generate message shown to user
     processSendCoinsReturn(prepareStatus,