Get rid of a few trivial compiler warnings. (#4312)

src/tree/updater_basemaker-inl.h

@@ -331,7 +331,6 @@ class BaseMaker: public TreeUpdater {
                            std::vector< std::vector<TStats> > *p_thread_temp,
                            std::vector<TStats> *p_node_stats) {
     std::vector< std::vector<TStats> > &thread_temp = *p_thread_temp;
-    const MetaInfo &info = fmat.Info();
     thread_temp.resize(omp_get_max_threads());
     p_node_stats->resize(tree.param.num_nodes);
 #pragma omp parallel

src/tree/updater_colmaker.cc

@@ -245,7 +245,6 @@ class ColMaker: public TreeUpdater {
                                  DMatrix *p_fmat,
                                   const std::vector<GradientPair> &gpair) {
       // TODO(tqchen): double check stats order.
-      const MetaInfo& info = p_fmat->Info();
       const bool ind = col.size() != 0 && col[0].fvalue == col[col.size() - 1].fvalue;
       bool need_forward = param_.NeedForwardSearch(p_fmat->GetColDensity(fid), ind);
       bool need_backward = param_.NeedBackwardSearch(p_fmat->GetColDensity(fid), ind);

src/tree/updater_skmaker.cc

@@ -140,7 +140,7 @@ class SketchMaker: public BaseMaker {
         this->UpdateSketchCol(gpair, batch[fidx], tree,
                               node_stats_,
                               fidx,
-                              batch[fidx].size() == nrows,
+                              static_cast<size_t>(batch[fidx].size()) == nrows,
                               &thread_sketch_[omp_get_thread_num()]);
       }
     }