Thread-safe prediction by making the prediction cache thread-local. (#5853)

Co-authored-by: Jiaming Yuan <jm.yuan@outlook.com>

src/tree/updater_quantile_hist.cc

@@ -1384,6 +1384,5 @@ XGBOOST_REGISTER_TREE_UPDATER(QuantileHistMaker, "grow_quantile_histmaker")
     []() {
       return new QuantileHistMaker();
     });
-
 }  // namespace tree
 }  // namespace xgboost