backend.c: do not try to explicitly clean up the libinput backend

Since libinput is an optional dependency the libinput backend is possibly undeclared.
wlr_backend_destroy(backend) below will clean up the child libinput backend if any.

backend/backend.c

@@ -463,7 +463,6 @@ struct wlr_backend *wlr_backend_autocreate(struct wl_display *display) {
 		attempt_drm_backend(display, backend, multi->session);
 	if (!primary_drm) {
 		wlr_log(WLR_ERROR, "Failed to open any DRM device");
-		wlr_backend_destroy(libinput);
 		wlr_session_destroy(multi->session);
 		wlr_backend_destroy(backend);
 		return NULL;