[data] remove orphaned Preprocessor doc code (#57872)

## Description
This removes an orphaned code file that was previously used by the
Preprocessor User Guide.

## Related issues
Corresponding User Guide was removed in #44006.
Closes #57867.

## Additional details
This test started failing because of the new `XGBoostTrainer` API
enabled by default with Ray Train V2. Rather than update the snippet,
removing this code instead.

Signed-off-by: Matthew Deng <matthew.j.deng@gmail.com>

Author: matthewdeng