backend/drm: remove wlr_drm_renderer.backend

This is unused.

backend/drm/renderer.c

@@ -13,8 +13,6 @@
 
 bool init_drm_renderer(struct wlr_drm_backend *drm,
 		struct wlr_drm_renderer *renderer) {
-	renderer->backend = drm;
-
 	renderer->wlr_rend = renderer_autocreate_with_drm_fd(drm->fd);
 	if (!renderer->wlr_rend) {
 		wlr_log(WLR_ERROR, "Failed to create renderer");

include/backend/drm/renderer.h

@@ -13,8 +13,6 @@ struct wlr_drm_plane;
 struct wlr_buffer;
 
 struct wlr_drm_renderer {
-	struct wlr_drm_backend *backend;
-
 	struct wlr_renderer *wlr_rend;
 	struct wlr_allocator *allocator;
 };