Fix clippy::needless_update lint (#5907)

This lint was introduced while merging #5980. Because the `updater_id`
and `updater_gen` fields are no longer part of `InstanceRuntimeState`,
the struct update syntax now doesn't provide any fields and Clippy asks
us to remove it.

nexus/db-queries/src/db/datastore/instance.rs

@@ -1359,7 +1359,6 @@ mod tests {
                     propolis_id: Some(active_vmm.id),
                     dst_propolis_id: Some(target_vmm.id),
                     migration_id: Some(migration.id),
-                    ..snapshot.instance.runtime_state.clone()
                 },
             )
             .await