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/vst2.h

@@ -68,7 +68,6 @@ class Vst2Bridge : public HostBridge {
     void run() override;
 
     void handle_x11_events() override;
-    void handle_win32_events() override;
 
     /**
      * Forward the host callback made by the plugin to the host and return the