Small cleanup for the model types.

[trivialfis]

• Remove binary-IO related code.
• Split up the approx tree SHAP implementation.

[Copilot]

Pull Request Overview

This PR performs a cleanup of the tree model implementation by removing deprecated binary IO code and refactoring the approximate SHAP implementation. The changes modernize the codebase by eliminating legacy code and improving code organization.

• Removes binary IO related code from TreeParam struct and RegTree class
• Moves the approximate SHAP calculation from RegTree to a standalone function in the treeshap module
• Updates header dependencies and function calls to use the new SHAP implementation

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/tree/tree_model.cc	Removes parameter registration, adds JSON serialization methods, and removes the approximate SHAP method
src/predictor/treeshap.h	New header file declaring the standalone approximate SHAP function
src/predictor/treeshap.cc	Implements the moved approximate SHAP calculation with updated function signature
src/predictor/cpu_treeshap.h	Deleted file - functionality moved to treeshap.h
src/predictor/cpu_predictor.cc	Updates include and function call to use new SHAP implementation
include/xgboost/tree_model.h	Removes binary IO code, deprecated fields, and the approximate SHAP method declaration

Comments suppressed due to low confidence (3)

src/predictor/treeshap.h:4

• The header guard name 'XGBOOST_PREDICTOR_CPU_TREESHAP_H_' contains 'CPU' but the file is named 'treeshap.h'. The guard should be 'XGBOOST_PREDICTOR_TREESHAP_H_' to match the filename.

#ifndef XGBOOST_PREDICTOR_CPU_TREESHAP_H_

src/predictor/treeshap.h:34

• The header guard closing comment 'XGBOOST_PREDICTOR_CPU_TREESHAP_H_' should match the opening guard and be 'XGBOOST_PREDICTOR_TREESHAP_H_' to be consistent with the filename.

#endif  // XGBOOST_PREDICTOR_CPU_TREESHAP_H_

src/predictor/treeshap.cc:18

• The variable type has changed from 'unsigned' to 'bst_feature_t' but this may cause type mismatch issues. The original code used 'unsigned split_index = 0' and later assigns 'tree[nid].SplitIndex()' which likely returns an unsigned type.

  bst_feature_t split_index = 0;

[trivialfis]

cc @rongou .