Use the new approach for creating plugin factory

Directly serializing and deserializing into objects was and more
boilerplate heavy (since we now need two implementations even though we
only use one), and also much less flexible because we can't wrap
payloads in structs or provide optional values that way.

src/wine-host/bridges/vst3.h

@@ -91,13 +91,6 @@ class Vst3Bridge : public HostBridge {
      */
     Vst3Sockets<Win32Thread> sockets;
 
-    /**
-     * A plugin factory copied from the Windows VST3 plugin during
-     * initialization. The native VST3 plugin will request a copy of this
-     * information during its initialization.
-     */
-    std::unique_ptr<YaPluginFactory> plugin_factory;
-
     /**
      * Used to assign unique identifier to instances created for
      * `IPluginFactory::createInstance()`.