Do not disconnect if drop resource groups

[nolouch]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. run workload like sysbench

2. drop the resource group which is used by sysbench

mysql> ALTER USER 'user1' RESOURCE GROUP default;
Query OK, 0 rows affected (0.06 sec)

mysql> select * from information_schema.resource_groups;
+---------+------------+----------+-----------+-------------+------------+
| NAME    | RU_PER_SEC | PRIORITY | BURSTABLE | QUERY_LIMIT | BACKGROUND |
+---------+------------+----------+-----------+-------------+------------+
| default | UNLIMITED  | MEDIUM   | YES       | NULL        | NULL       |
| rg1     | 100000000  | HIGH     | NO        | NULL        | NULL       |
+---------+------------+----------+-----------+-------------+------------+
2 rows in set (0.06 sec)

mysql> DROP resource group rg1;
Query OK, 0 rows affected (0.56 sec)

2. What did you expect to see? (Required)

sysbench no disconnect

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

[nolouch]

PM: we can let the old connection use the default group in this situation.