Fix "Minimise at startup" on Windows

This option didn't work properly when "Hide window to system
tray when minimised" was also enabled.

I don't see a particular reason why Windows shouldn't be calling
hideWindow() like all other platforms. hideWindow() takes care of
choosing the correct minimisation mode based on the user's settings.

src/core/Bootstrap.cpp

@@ -101,11 +101,7 @@ namespace Bootstrap
     {
         // start minimized if configured
         if (config()->get(Config::GUI_MinimizeOnStartup).toBool()) {
-#ifdef Q_OS_WIN
-            mainWindow.showMinimized();
-#else
             mainWindow.hideWindow();
-#endif
         } else {
             mainWindow.bringToFront();
         }