Demos: Fix AudioPluginDemo Steinberg VST3 Validator failure when disabling buses

examples/Plugins/AudioPluginDemo.h

@@ -204,10 +204,6 @@ public:
         if (! mainInput.isDisabled() && mainInput != mainOutput)
             return false;
 
-        // do not allow disabling the main buses
-        if (mainOutput.isDisabled())
-            return false;
-
         // only allow stereo and mono
         if (mainOutput.size() > 2)
             return false;