Remove stub mirror driver code

We don't have any meaningful code for the mirror driver, so
remove the confusing stub and interface for it.
diff --git a/win/rfb_win32/SDisplay.h b/win/rfb_win32/SDisplay.h
index 6aac59a..70208a7 100644
--- a/win/rfb_win32/SDisplay.h
+++ b/win/rfb_win32/SDisplay.h
@@ -96,9 +96,8 @@
       static BoolParameter removePattern;
       static BoolParameter disableEffects;
 
-      // -=- Use by VNC Config to determine whether hooks, driver, etc are available
+      // -=- Use by VNC Config to determine whether hooks are available
       static bool areHooksAvailable();
-      static bool isDriverAvailable();
 
 
     protected: