Remove update prediction cache from predictors. (#5312)

Move this function into gbtree, and uses only updater for doing so. As now the predictor knows exactly how many trees to predict, there's no need for it to update the prediction cache.

src/common/hist_util.h

@@ -199,7 +199,7 @@ class CutsBuilder {
   }
 
   void AddCutPoint(WQSketch::SummaryContainer const& summary, int max_bin) {
-    int required_cuts = std::min(static_cast<int>(summary.size), max_bin);
+    size_t required_cuts = std::min(summary.size, static_cast<size_t>(max_bin));
     for (size_t i = 1; i < required_cuts; ++i) {
       bst_float cpt = summary.data[i].value;
       if (i == 1 || cpt > p_cuts_->cut_values_.back()) {