Change default value of senttxstore to false

Currently this data is stored in plaintext on disk, which is less than ideal.
Users should explicitly opt-in to this feature. In the future, we plan to encrypt
the senttxstore. When that happens, we could default this feature to on again,
as long as the user has specified a PIN/password for the SilentDragon installation.

src/settings.cpp

@@ -160,7 +160,7 @@ void Settings::setAllowCustomFees(bool allow) {
 
 bool Settings::getSaveZtxs() {
     // Load from the QT Settings. 
-    return QSettings().value("options/savesenttx", true).toBool();
+    return QSettings().value("options/savesenttx", false).toBool();
 }
 
 void Settings::setSaveZtxs(bool save) {