## What changes were proposed in this pull request?
Currently "EXPLAIN DESC TABLE" is special cased and outputs a single row relation as following.
Current output:
```sql
spark-sql> EXPLAIN DESCRIBE TABLE t;
== Physical Plan ==
*(1) Scan OneRowRelation[]
```
This is not consistent with how we handle explain processing for other commands. In this PR, the inconsistency is handled by removing the special handling for "describe table".
After change:
```sql
spark-sql> EXPLAIN DESC EXTENDED t
== Physical Plan ==
Execute DescribeTableCommand
+- DescribeTableCommand `t`, true
```
## How was this patch tested?
Added new tests in SQLQueryTestSuite.
Closes#24427 from dilipbiswal/describe_table_explain2.
Authored-by: Dilip Biswal <dbiswal@us.ibm.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>
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