Revert "Expose PluginInfo index (for VST3 shells)"

This reverts commit 04e5ea0f86.

libs/ardour/ardour/plugin.h

@@ -468,7 +468,6 @@ public:
 	bool plugintype_name_ambiguity;
 
 	std::string unique_id;
-	uint32_t index; //< used for LADSPA & VST3, index in module
 
 	virtual PluginPtr load (Session& session) = 0;
 
@@ -499,6 +498,7 @@ public:
 
 protected:
 	friend class PluginManager;
+	uint32_t index; //< used for LADSPA, index in module
 };
 
 } // namespace ARDOUR