output: remove wlr_output_impl.transform

The backend doesn't need to handle transform changes, since everything is done
in software. In fact, all of the implementations were all identical and just
set the transform.

We could add support for hardware transforms, but:

- This would require a different field (something like hardware_transform)
- Not all combinations are possible because there often are hardware
  limitations
- The Wayland protocol isn't ready for this (in particular xdg-output, see [1])

This belongs to a different patch series anyway.

[1]: https://patchwork.freedesktop.org/series/52324/

backend/noop/output.c

@@ -12,11 +12,6 @@ static struct wlr_noop_output *noop_output_from_output(
 	return (struct wlr_noop_output *)wlr_output;
 }
 
-static void output_transform(struct wlr_output *wlr_output,
-		enum wl_output_transform transform) {
-	// empty
-}
-
 static bool output_set_custom_mode(struct wlr_output *wlr_output,
 		int32_t width, int32_t height, int32_t refresh) {
 	wlr_output_update_custom_mode(wlr_output, width, height, refresh);
@@ -42,7 +37,6 @@ static void output_destroy(struct wlr_output *wlr_output) {
 }
 
 static const struct wlr_output_impl output_impl = {
-	.transform = output_transform,
 	.set_custom_mode = output_set_custom_mode,
 	.destroy = output_destroy,
 	.attach_render = output_attach_render,