Compiler warnings (#6286)

* Fix warnings for json.h

* Fix warnings for metric.h

* Fix warnings for updater_quantile_hist.cc.

* Fix warnings for updater_histmaker.cc.

Co-authored-by: Hyunsu Cho <chohyu01@cs.washington.edu>

include/xgboost/json.h

@@ -250,7 +250,7 @@ class JsonNull : public Value {
  public:
   JsonNull() : Value(ValueKind::kNull) {}
   JsonNull(std::nullptr_t) : Value(ValueKind::kNull) {}  // NOLINT
-  JsonNull(JsonNull&& that) : Value(ValueKind::kNull) {}
+  JsonNull(JsonNull&&) : Value(ValueKind::kNull) {}
 
   void Save(JsonWriter* writer) override;
 

include/xgboost/metric.h

@@ -34,7 +34,7 @@ class Metric : public Configurable {
    * \param args arguments to the objective function.
    */
   virtual void Configure(
-      const std::vector<std::pair<std::string, std::string> >& args) {}
+      const std::vector<std::pair<std::string, std::string> >&) {}
   /*!
    * \brief Load configuration from JSON object
    * By default, metric has no internal configuration;