radeon: drop unused constant_cliprect field

src/mesa/drivers/dri/radeon/radeon_common.c

@@ -305,7 +305,6 @@ void radeon_draw_buffer(struct gl_context *ctx, struct gl_framebuffer *fb)
 			offset = rrb->draw_offset;
 			rrbColor = rrb;
 		}
-		radeon->constant_cliprect = GL_TRUE;
 	}
 
 	if (rrbColor == NULL)

src/mesa/drivers/dri/radeon/radeon_common_context.h

@@ -438,7 +438,6 @@ struct radeon_context {
    struct radeon_debug debug;
 
   drm_clip_rect_t fboRect;
-  GLboolean constant_cliprect; /* use for FBO or DRI2 rendering */
   GLboolean front_cliprects;
 
    /**