gallium: Remove pipe_screen::update_buffer.

It has no user after the removal of st_public.  Plus, it has never been
implemented by a pipe driver or winsys.

src/gallium/auxiliary/util/u_simple_screen.h

@@ -52,11 +52,6 @@ struct pipe_winsys
    /** Returns name of this winsys interface */
    const char *(*get_name)( struct pipe_winsys *ws );
 
-   /**
-    * Do any special operations to ensure buffer size is correct
-    */
-   void (*update_buffer)( struct pipe_winsys *ws,
-                          void *context_private );
    /**
     * Do any special operations to ensure frontbuffer contents are
     * displayed, eg copy fake frontbuffer.

src/gallium/include/pipe/p_screen.h

@@ -169,13 +169,6 @@ struct pipe_screen {
 					       unsigned bytes,
 					       unsigned bind_flags);
 
-   /**
-    * Do any special operations to ensure buffer size is correct
-    * \param context_private  the private data of the calling context
-    */
-   void (*update_buffer)( struct pipe_screen *ws,
-                          void *context_private );
-
    /**
     * Do any special operations to ensure frontbuffer contents are
     * displayed, eg copy fake frontbuffer.