Clean up warnings (#6325)

src/gbm/gblinear_model.h

@@ -49,7 +49,7 @@ class GBLinearModel : public Model {
  public:
   explicit GBLinearModel(LearnerModelParam const* learner_model_param) :
       learner_model_param {learner_model_param} {}
-  void Configure(Args const &cfg) { }
+  void Configure(Args const &) { }
 
   // weight for each of feature, bias is the last one
   std::vector<bst_float> weight;