qt: Don't log to console by default

Default `-printtoconsole` to false for the GUI. GUI programs should not
print to the console unnecessarily. For example, when launched by the
window manager, the output might end up in the X session log file,
resulting in duplicate logging. On Windows, it is pointless as well
because bitcoin-qt isn't a console application.

src/qt/bitcoin.cpp

@@ -403,6 +403,10 @@ void BitcoinApplication::startThread()
 
 void BitcoinApplication::parameterSetup()
 {
+    // Default printtoconsole to false for the GUI. GUI programs should not
+    // print to the console unnecessarily.
+    gArgs.SoftSetBoolArg("-printtoconsole", false);
+
     m_node.initLogging();
     m_node.initParameterInteraction();
 }