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/wayland/output.c

@@ -86,12 +86,6 @@ static bool output_commit(struct wlr_output *wlr_output) {
 	return true;
 }
 
-static void output_transform(struct wlr_output *wlr_output,
-		enum wl_output_transform transform) {
-	struct wlr_wl_output *output = get_wl_output_from_output(wlr_output);
-	output->wlr_output.transform = transform;
-}
-
 static bool output_set_cursor(struct wlr_output *wlr_output,
 		struct wlr_texture *texture, int32_t scale,
 		enum wl_output_transform transform,
@@ -226,7 +220,6 @@ static bool output_schedule_frame(struct wlr_output *wlr_output) {
 
 static const struct wlr_output_impl output_impl = {
 	.set_custom_mode = output_set_custom_mode,
-	.transform = output_transform,
 	.destroy = output_destroy,
 	.attach_render = output_attach_render,
 	.commit = output_commit,