Remove some warnings from clang. (#7533)

* Unused variable.
* Unnecessary virtual function.

src/c_api/c_api_utils.h

@@ -165,9 +165,9 @@ inline float GetMissing(Json const &config) {
 
 // Safe guard some global variables from being changed by XGBoost.
 class XGBoostAPIGuard {
+#if defined(XGBOOST_USE_CUDA)
   int32_t device_id_ {0};
 
-#if defined(XGBOOST_USE_CUDA)
   void SetGPUAttribute();
   void RestoreGPUAttribute();
 #else

src/tree/updater_quantile_hist.h

@@ -166,11 +166,8 @@ class QuantileHistMaker: public TreeUpdater {
       builder_monitor_.Init("Quantile::Builder");
     }
     // update one tree, growing
-    virtual void Update(const GHistIndexMatrix& gmat,
-                        const ColumnMatrix& column_matrix,
-                        HostDeviceVector<GradientPair>* gpair,
-                        DMatrix* p_fmat,
-                        RegTree* p_tree);
+    void Update(const GHistIndexMatrix& gmat, const ColumnMatrix& column_matrix,
+                HostDeviceVector<GradientPair>* gpair, DMatrix* p_fmat, RegTree* p_tree);
 
     bool UpdatePredictionCache(const DMatrix* data,
                                linalg::VectorView<float> out_preds);