refactor: Remove unused Parquet indexes

[coastalwhite]

This is not used anywhere in the code except the tests.

[codecov]

Codecov Report

Attention: Patch coverage is 80.55556% with 7 lines in your changes missing coverage. Please review.

Project coverage is 80.54%. Comparing base (a601ccf) to head (9b97436).
Report is 21 commits behind head on main.

Files	Patch %	Lines
crates/polars-parquet/src/parquet/write/page.rs	50.00%	4 Missing ⚠️
crates/polars-parquet/src/parquet/page/mod.rs	84.21%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18329      +/-   ##
==========================================
+ Coverage   80.35%   80.54%   +0.19%     
==========================================
  Files        1501     1491      -10     
  Lines      199236   198349     -887     
  Branches     2837     2837              
==========================================
- Hits       160087   159769     -318     
+ Misses      38623    38054     -569     
  Partials      526      526              

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