[MINOR][PYTHON] Remove explain(True) in test_udf.py

## What changes were proposed in this pull request?

Not a big deal but it bugged me. This PR removes printing out plans in PySpark UDF tests.

Before:

```
Running tests...
----------------------------------------------------------------------
Setting default log level to "WARN".
To adjust logging level use sc.setLogLevel(newLevel). For SparkR, use setLogLevel(newLevel).
== Parsed Logical Plan ==
GlobalLimit 1
+- LocalLimit 1
   +- Project [id#668L, <lambda>(id#668L) AS copy#673]
      +- Sort [id#668L ASC NULLS FIRST], true
         +- Range (0, 10, step=1, splits=Some(4))

== Analyzed Logical Plan ==
id: bigint, copy: int
GlobalLimit 1
+- LocalLimit 1
   +- Project [id#668L, <lambda>(id#668L) AS copy#673]
      +- Sort [id#668L ASC NULLS FIRST], true
         +- Range (0, 10, step=1, splits=Some(4))

== Optimized Logical Plan ==
GlobalLimit 1
+- LocalLimit 1
   +- Project [id#668L, pythonUDF0#676 AS copy#673]
      +- BatchEvalPython [<lambda>(id#668L)], [id#668L, pythonUDF0#676]
         +- Range (0, 10, step=1, splits=Some(4))

== Physical Plan ==
CollectLimit 1
+- *(2) Project [id#668L, pythonUDF0#676 AS copy#673]
   +- BatchEvalPython [<lambda>(id#668L)], [id#668L, pythonUDF0#676]
      +- *(1) Range (0, 10, step=1, splits=4)

...........................................
----------------------------------------------------------------------
Ran 43 tests in 19.777s
```

After:

```
Running tests...
----------------------------------------------------------------------
Setting default log level to "WARN".
To adjust logging level use sc.setLogLevel(newLevel). For SparkR, use setLogLevel(newLevel).
...........................................
----------------------------------------------------------------------
Ran 43 tests in 25.201s
```

## How was this patch tested?

N/A

Closes apache#24661 from HyukjinKwon/remove-explain-in-test.

Authored-by: HyukjinKwon <[email protected]>
Signed-off-by: HyukjinKwon <[email protected]>

Author: HyukjinKwon

python/pyspark/sql/tests/test_udf.py

@@ -326,7 +326,6 @@ def test_udf_with_order_by_and_limit(self):
         my_copy = udf(lambda x: x, IntegerType())
         df = self.spark.range(10).orderBy("id")
         res = df.select(df.id, my_copy(df.id).alias("copy")).limit(1)
-        res.explain(True)
         self.assertEqual(res.collect(), [Row(id=0, copy=0)])
 
     def test_udf_registration_returns_udf(self):