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>
1 file changed