[SPARK-48482][PYTHON][FOLLOWUP] dropDuplicates and dropDuplicatesWIthinWatermark should accept named parameter

[WweiL]

What changes were proposed in this pull request?

https://github.com/apache/spark/commit/560c08332b35941260169124b4f522bdc82b84d8 unintentionally made dropDuplicates(subset=["col"]) doesn't work, this patches this scenario.

Why are the changes needed?

Bug fix

Does this PR introduce any user-facing change?

No

How was this patch tested?

Unit test

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

No

[WweiL]

cc @HyukjinKwon @allisonwang-db @itholic PTAL!

[allisonwang-db]

My general comment here is that we should be opinionated and only have one way to perform certain operations. After this change, users now have two identical ways to drop duplicates:

1. dropDuplicates("c1", "c2")
2. dropDuplicates(["c1", "c2"])

Which one should users choose?

P.S. Pandas / Pandas on Spark uses the subset=["c1", "c2"] pattern (see: https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.drop_duplicates.html and https://spark.apache.org/docs/latest/api/python/reference/pyspark.pandas/api/pyspark.pandas.DataFrame.drop_duplicates.html). It could be confusing to make the PySpark API different.