Remove public access to tree model param. (#8902)

* Make tree model param a private member.
* Number of features and targets are immutable after construction.

This is to reduce the number of places where we can run configuration.

tests/cpp/tree/test_prune.cc

@@ -32,8 +32,7 @@ TEST(Updater, Prune) {
   auto ctx = CreateEmptyGenericParam(GPUIDX);
 
   // prepare tree
-  RegTree tree = RegTree();
-  tree.param.UpdateAllowUnknown(cfg);
+  RegTree tree = RegTree{1u, kCols};
   std::vector<RegTree*> trees {&tree};
   // prepare pruner
   TrainParam param;