[SPARK-7631] [SQL] treenode argString should not print children

spark-sql>
> explain extended
> select * from (
> select key from src union all
> select key from src) t;

now the spark plan will print children in argString
```
== Physical Plan ==
Union[ HiveTableScan key#1, (MetastoreRelation default, src, None), None,
HiveTableScan key#3, (MetastoreRelation default, src, None), None]
HiveTableScan key#1, (MetastoreRelation default, src, None), None
HiveTableScan key#3, (MetastoreRelation default, src, None), None
```

after this patch:
```
== Physical Plan ==
Union
 HiveTableScan [key#1], (MetastoreRelation default, src, None), None
 HiveTableScan [key#3], (MetastoreRelation default, src, None), None
```

I have tested this locally

Author: scwf <wangfei1@huawei.com>

Closes #6144 from scwf/fix-argString and squashes the following commits:

1a642e0 [scwf] fix treenode argString