TST/CLN: remove redundant to_json test (pandas-dev#40577)

Author: mzeitlin11

pandas/tests/io/json/test_pandas.py

@@ -1730,11 +1730,6 @@ def test_to_s3(self, s3_resource, s3so):
             timeout -= 0.1
             assert timeout > 0, "Timed out waiting for file to appear on moto"
 
-    def test_json_pandas_na(self):
-        # GH 31615
-        result = DataFrame([[pd.NA]]).to_json()
-        assert result == '{"0":{"0":null}}'
-
     def test_json_pandas_nulls(self, nulls_fixture, request):
         # GH 31615
         if isinstance(nulls_fixture, Decimal):