refactor: Don't run streaming group-by in partitionable gb

[ritchie46]

@stinodego FYI

[stinodego]

Great. That means that POLARS_NO_STREAMING_GROUPBY is no longer used, right? There's a test that explicitly sets this, we can remove the monkeypatch there.

[codspeed-hq]

CodSpeed Performance Report

Merging #15611 will degrade performances by 15.23%

_{Comparing streaming (406b68e) with main (c068e76)}

Summary

⚡ 1 improvements
❌ 1 regressions
✅ 20 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	streaming	Change
⚡	test_h2oai_groupby_q1	6.5 s	5.3 s	+24.08%
❌	test_h2oai_groupby_q4	5.7 s	6.7 s	-15.23%

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.25%. Comparing base (c068e76) to head (406b68e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15611      +/-   ##
==========================================
+ Coverage   81.20%   81.25%   +0.04%     
==========================================
  Files        1371     1371              
  Lines      175822   175772      -50     
  Branches     2531     2531              
==========================================
+ Hits       142782   142824      +42     
+ Misses      32563    32471      -92     
  Partials      477      477              

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