dri: Remove driRenderbuffer::backBuffer field

The tdfx driver was the only user.

Acked-by: Kristian Høgsberg <krh@bitplanet.net>
Acked-by: Marek Olšák <maraeo@gmail.com>
Acked-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Acked-by: Jakob Bornecrantz <jakob@vmware.com>
Acked-by: Dave Airlie <airlied@redhat.com>
Build-Tested-by: Jakob Bornecrantz <jakob@vmware.com>
Tested-by: Eugeni Dodonov <eugeni.dodonov@intel.com>

src/mesa/drivers/dri/common/drirenderbuffer.h

@@ -52,14 +52,6 @@ typedef struct {
     * r200Renderbuffer class, derived from this class...  not a huge deal.
     */
    GLboolean depthHasSurface;
-
-   /**
-    * A handy flag to know if this is the back color buffer.
-    * 
-    * \note
-    * This is currently only used by tdfx.
-    */
-   GLboolean backBuffer;
 } driRenderbuffer;