undistribute_table doesn't drop sequences on workers

```sql
CREATE TABLE t(
id bigserial
);
SELECT create_distributed_table('t', 'id');
SELECT undistribute_table('t');
```

On worker:
```
> \d
List of relations
Schema │ Name │ Type │ Owner
────────┼──────────────┼──────────┼───────
public │ citus_tables │ view │ jelte
public │ t_id_seq │ sequence │ jelte
```

Originally reported as a comment in #6694