refactor: Remove even more parquet multiscan handling

[coastalwhite]

No description provided.

[coastalwhite]

@orlp this should also partially fix the slowdown on q7 I believe

[codecov]

Codecov Report

Attention: Patch coverage is 84.22819% with 47 lines in your changes missing coverage. Please review.

Project coverage is 80.03%. Comparing base (db551fb) to head (1d37494).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
...polars-stream/src/nodes/io_sources/parquet/init.rs	80.50%	31 Missing ⚠️
...c/nodes/io_sources/parquet/row_group_data_fetch.rs	83.75%	13 Missing ⚠️
crates/polars-io/src/parquet/read/predicates.rs	93.75%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #21601      +/-   ##
==========================================
+ Coverage   79.75%   80.03%   +0.28%     
==========================================
  Files        1604     1604              
  Lines      231527   231328     -199     
  Branches     2639     2639              
==========================================
+ Hits       184658   185147     +489     
+ Misses      46260    45572     -688     
  Partials      609      609              

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