refactor(rust!): Remove incremental read based batched CSV reader

[nameexhaustion]

Removes code that was previously disabled by https://github.com/pola-rs/polars/pull/16231

• Removes BatchedCsvReaderRead
• Renames BatchedCsvReaderMmap -> BatchedCsvReader, fn batched_mmap -> fn batched etc.
  • I like this because it shortens the name, and hides mmap as an implementation detail

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 95.65217% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 80.87%. Comparing base (d7d720a) to head (1112d3f).

Files	Patch %	Lines
crates/polars-io/src/csv/read/reader.rs	83.33%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #16259 +/- ## ========================================== - Coverage 80.88% 80.87% -0.01% ========================================== Files 1394 1393 -1 Lines 179488 179159 -329 Branches 2913 2913 ========================================== - Hits 145173 144901 -272 + Misses 33812 33755 -57 Partials 503 503 ```

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[ritchie46]

I like this because it shortens the name, and hides mmap as an implementation detail

Yeap, agree. :+1: