Use cudf::distinct in Java binding

[ttnghia]

Java binding has dropDuplicates API to remove duplicate rows from a table. Previously it has been implemented by sorting the table then calling to cudf::unique. This PR changes the implementation to use cudf::distinct directly, which can significantly improve performance by avoiding sorting the input table.

[revans2]

It would have been nice to have not broken an existing public API. But it does not appear to be used right now, and there are semantic changes to the output too, so I am not as concerned.

[codecov]

Codecov Report

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[ttnghia]

@gpucibot merge