refactor(rust): Temporarily disable common subplan elim for new-streaming

[orlp]

The goal is to re-enable this ASAP by translating the cache nodes to multiplexer nodes (or something else), but this fixes a todo!() branch for now.

[codecov]

Codecov Report

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

Project coverage is 78.92%. Comparing base (db1684d) to head (cbab50f).
Report is 25 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-lazy/src/frame/mod.rs	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20374      +/-   ##
==========================================
- Coverage   79.10%   78.92%   -0.18%     
==========================================
  Files        1572     1560      -12     
  Lines      219958   219691     -267     
  Branches     2465     2471       +6     
==========================================
- Hits       173991   173390     -601     
- Misses      45399    45733     +334     
  Partials      568      568              

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