Remove EXPERIMENTAL_OUTPUT_DISPLAY...

... Maybe a good feature idea, but the implementation pollutes the code in
too many places.

It's a special case of the more general idea of many-to-one associations
between screen rectangles and track objects.  More generalized ways to
accommodate that should be sought.

src/prefs/GUIPrefs.cpp

@@ -204,11 +204,6 @@ void GUIPrefs::PopulateOrExchange(ShuttleGui & S)
       S.TieCheckBox(_("B&lend system and Audacity theme"),
                     wxT("/GUI/BlendThemes"),
                     true);
-#ifdef EXPERIMENTAL_OUTPUT_DISPLAY
-      S.TieCheckBox(_("&Display a mono channel as virtual stereo"),
-                    wxT("/GUI/MonoAsVirtualStereo"),
-                    false);
-#endif
    }
    S.EndStatic();
 }