window.c: Drop timestamp argument to input_set_pointer_image()

We need the serial to set a pointer image and we track that internally.
diff --git a/clients/window.h b/clients/window.h
index 1db5eb0..653eaeb 100644
--- a/clients/window.h
+++ b/clients/window.h
@@ -364,7 +364,7 @@
 frame_create(struct window *window, void *data);
 
 void
-input_set_pointer_image(struct input *input, uint32_t time, int pointer);
+input_set_pointer_image(struct input *input, int pointer);
 
 void
 input_get_position(struct input *input, int32_t *x, int32_t *y);