Drop the deprecated binary model for gblinear. (#11553)

Author: trivialfis

src/gbm/gblinear.cc

@@ -104,11 +104,11 @@ class GBLinear : public GradientBooster {
 
   bool ModelFitted() const override { return BoostedRounds() != 0; }
 
-  void Load(dmlc::Stream* fi) override {
-    model_.Load(fi);
+  void Load(dmlc::Stream*) override {
+    LOG(FATAL) << "The deprecated binary model has been removed";
   }
-  void Save(dmlc::Stream* fo) const override {
-    model_.Save(fo);
+  void Save(dmlc::Stream*) const override {
+    LOG(FATAL) << "The deprecated binary model has been removed";
   }
 
   void SaveModel(Json* p_out) const override {

src/gbm/gblinear_model.h

@@ -1,5 +1,5 @@
-/*!
- * Copyright 2018-2019 by Contributors
+/**
+ * Copyright 2018-2025, XGBoost Contributors
  */
 #pragma once
 #include <dmlc/io.h>
@@ -71,17 +71,6 @@ class GBLinearModel : public Model {
   void SaveModel(Json *p_out) const override;
   void LoadModel(Json const &in) override;
 
-  // save the model to file
-  void Save(dmlc::Stream *fo) const {
-    fo->Write(&param_, sizeof(param_));
-    fo->Write(weight);
-  }
-  // load model from file
-  void Load(dmlc::Stream *fi) {
-    CHECK_EQ(fi->Read(&param_, sizeof(param_)), sizeof(param_));
-    fi->Read(&weight);
-  }
-
   // model bias
   inline bst_float *Bias() {
     return &weight[learner_model_param->num_feature *