Fix compiler warnings. (#8022)

- Remove/fix unused parameters
- Remove deprecated code in rabit.
- Update dmlc-core.

src/objective/hinge.cu

@@ -27,9 +27,7 @@ class HingeObj : public ObjFunction {
   void Configure(Args const&) override {}
   ObjInfo Task() const override { return ObjInfo::kRegression; }
 
-  void GetGradient(const HostDeviceVector<bst_float> &preds,
-                   const MetaInfo &info,
-                   int iter,
+  void GetGradient(const HostDeviceVector<bst_float> &preds, const MetaInfo &info, int /*iter*/,
                    HostDeviceVector<GradientPair> *out_gpair) override {
     CHECK_NE(info.labels.Size(), 0U) << "label set cannot be empty";
     CHECK_EQ(preds.Size(), info.labels.Size())
@@ -86,7 +84,7 @@ class HingeObj : public ObjFunction {
     auto& out = *p_out;
     out["name"] = String("binary:hinge");
   }
-  void LoadConfig(Json const& in) override {}
+  void LoadConfig(Json const &) override {}
 };
 
 // register the objective functions