[SPARK-54278][PYTHON][TESTS] Simplify pyspark/util.py doctests by removing Python 3.9+ condition

### What changes were proposed in this pull request?

This PR aims to simplify `pyspark/util.py` doctests by removing Python 3.9+ condition.

### Why are the changes needed?

Since Apache Spark 4.1.0 dropped `Python 3.9` support, we can assume that this doctests are Python 3.10+ always.
- apache#51416
- apache#51259
- apache#52974

### Does this PR introduce _any_ user-facing change?

No, this is a test change.

### How was this patch tested?

Pass the CIs.

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes apache#52975 from dongjoon-hyun/SPARK-54278.

Authored-by: Dongjoon Hyun <dongjoon@apache.org>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

Author: dongjoon-hyun

python/pyspark/util.py

@@ -918,7 +918,7 @@ def default_api_mode() -> str:
 
 
 if __name__ == "__main__":
-    if "pypy" not in platform.python_implementation().lower() and sys.version_info[:2] >= (3, 9):
+    if "pypy" not in platform.python_implementation().lower():
         import doctest
         import pyspark.util
         from pyspark.core.context import SparkContext