Move Win32 event handling to one place

Or technically, two, since the group bridge also does the same loop. We
no longer need special handling for VST2 and VST3 plugins, so we can
simplify things a bit here.

src/wine-host/bridges/vst3.h

@@ -232,7 +232,6 @@ class Vst3Bridge : public HostBridge {
     void run() override;
 
     void handle_x11_events() override;
-    void handle_win32_events() override;
 
     /**
      * Send a callback message to the host return the response. This is a