softpipe: remove unused quad_shade_stage()

Fixes the following Clang warning.

sp_quad_fs.c:60:1: warning: unused function 'quad_shade_stage' [-Wunused-function]
quad_shade_stage(struct quad_stage *qs)
^
1 warning generated.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Brian Paul <brianp@vmware.com>

src/gallium/drivers/softpipe/sp_quad_fs.c

@@ -55,14 +55,6 @@ struct quad_shade_stage
 };
 
 
-/** cast wrapper */
-static inline struct quad_shade_stage *
-quad_shade_stage(struct quad_stage *qs)
-{
-   return (struct quad_shade_stage *) qs;
-}
-
-
 /**
  * Execute fragment shader for the four fragments in the quad.
  * \return TRUE if quad is alive, FALSE if all four pixels are killed