[SPARK-43874][FOLLOWUP][TESTS] Enable GroupbyIndexTests.test_groupby_multiindex_columns

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

Follow-up for #42533.

### Why are the changes needed?

To enable test.

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

No.

### How was this patch tested?

The existing CI should pass.

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

No.

Closes #42941 from itholic/SPARK-43874-followup.

Authored-by: Haejoon Lee <haejoon.lee@databricks.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>

Author: itholic

python/pyspark/pandas/tests/groupby/test_index.py

@@ -40,10 +40,6 @@ def pdf(self):
     def psdf(self):
         return ps.from_pandas(self.pdf)
 
-    @unittest.skipIf(
-        LooseVersion(pd.__version__) >= LooseVersion("2.0.0"),
-        "TODO(SPARK-43555): Enable GroupByTests.test_groupby_multiindex_columns for pandas 2.0.0.",
-    )
     def test_groupby_multiindex_columns(self):
         pdf = pd.DataFrame(
             {