[SPARK-44289][FOLLOWUP] Cleanup doctest

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

This is followup for #42533 to remove meaningless import

### Why are the changes needed?

import numpy is not needed for doctest.

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

This impacts to user-facing documents, so it's sort of user-facing cleanup.

### How was this patch tested?

The existing CI should pass

Closes #42552 from itholic/test-followup.

Authored-by: itholic <haejoon.lee@databricks.com>
Signed-off-by: Ruifeng Zheng <ruifengz@apache.org>

Author: itholic

python/pyspark/pandas/groupby.py

@@ -4165,7 +4165,6 @@ def value_counts(
 
         Examples
         --------
-        >>> import numpy as np
         >>> df = ps.DataFrame({'A': [1, 2, 2, 3, 3, 3],
         ...                    'B': [1, 1, 2, 3, 3, np.nan]},
         ...                   columns=['A', 'B'])