Remove default constructor for Vst3PluginInstance

Seems weird to need this specifically so we can use the map overload
that creates a new instance when the key doesn't exist in the map. This
seems safer.

src/wine-host/bridges/vst3.h

@@ -61,8 +61,6 @@ struct Vst3PlugViewInterfaces {
  * `IPluginBase::initialize()`.
  */
 struct Vst3PluginInstance {
-    Vst3PluginInstance() noexcept;
-
     Vst3PluginInstance(Steinberg::IPtr<Steinberg::FUnknown> object) noexcept;
 
     /**