The cost trace should not print the plan ID

## Enhancement

The cost trace should not print the plan ID. Too many plan IDs will affect my test cases.

```
explain format = 'cost_trace' select * from t limit 1;
id estRows estCost costFormula task access object operator info
Limit_7 1.00 12.97 ((((((scan(1*logrowsize(16)*tikv_scan_factor(40.7)))*1.00)) + (net(1*rowsize(8)*tidb_kv_net_factor(3.96))))/15.00)*1.00) root offset:0, count:1
└─IndexReader_12 1.00 12.97 (((((scan(1*logrowsize(16)*tikv_scan_factor(40.7)))*1.00)) + (net(1*rowsize(8)*tidb_kv_net_factor(3.96))))/15.00)*1.00 root index:Limit_11
└─Limit_11 1.00 162.80 ((scan(1*logrowsize(16)*tikv_scan_factor(40.7)))*1.00) cop[tikv] offset:0, count:1
└─IndexFullScan_10 1.00 162.80 (scan(1*logrowsize(16)*tikv_scan_factor(40.7)))*1.00 cop[tikv] table:t, index:a(a) keep order:false, stats:pseudo
```