refactor(rust): Fix duplicate cols in new-streaming parquet prefilter

[nameexhaustion]

Regression from https://github.com/pola-rs/polars/pull/20203/files#diff-554fb85df2859feb204caf7a9f438fb3fef8adc8affacb4c41ccf094a5e40c0cL278

[coastalwhite]

This is my bad. I saw that we now had a Set, so the dedup could be removed. I wrongly assumed the sort_unstable was for the dedup.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.

Project coverage is 78.98%. Comparing base (6aaa98c) to head (33cfa69).
Report is 39 commits behind head on main.

Files with missing lines	Patch %	Lines
...polars-stream/src/nodes/io_sources/parquet/init.rs	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20419      +/-   ##
==========================================
- Coverage   78.99%   78.98%   -0.01%     
==========================================
  Files        1562     1562              
  Lines      220104   220108       +4     
  Branches     2486     2486              
==========================================
- Hits       173865   173852      -13     
- Misses      45666    45683      +17     
  Partials      573      573              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[nameexhaustion]

I wrongly assumed the sort_unstable was for the dedup.

Ah - that makes sense actually, I think I would have done the same 😄