reduce rule numbers if not set the role in the rule SQL

[nolouch]

Problem

 create placement policy retail_pos constraints="[+retail_region=fast]";
 CREATE TABLE t1 (a INT);
 ALTER TABLE t1  PLACEMENT POLICY=retail_pos;

The rules in PD side:

[
  {
    "group_id": "TiDB_DDL_90",
    "id": "table_rule_90_0",
    "index": 40,
    "start_key": "7480000000000000ff5a00000000000000f8",
    "end_key": "7480000000000000ff5b00000000000000f8",
    "role": "leader",
    "is_witness": false,
    "count": 1,
    "label_constraints": [
      {
        "key": "retail_region",
        "op": "in",
        "values": [
          "fast"
        ]
      },
      ....
    ],
    "create_timestamp": 1689589620
  },
  {
    "group_id": "TiDB_DDL_90",
    "id": "table_rule_90_1",
    "index": 40,
    "start_key": "7480000000000000ff5a00000000000000f8",
    "end_key": "7480000000000000ff5b00000000000000f8",
    "role": "voter",
    "is_witness": false,
    "count": 2,
    "label_constraints": [
      {
        "key": "retail_region",
        "op": "in",
        "values": [
          "fast"
        ]
      },
      ...
    ],
    "create_timestamp": 1689589620
  },
  {
    "group_id": "pd",
    "id": "default",
    "start_key": "",
    "end_key": "",
    "role": "voter",
    "is_witness": false,
    "count": 3
  }
]

There is no need to split into two rules because we do not need to distinguish roles, two rules perhaps exist some problems. such as location-labels may not work in tikv/pd#6637