[Breaking] Don't drop trees during DART prediction by default (#5115)

* Simplify DropTrees calling logic

* Add `training` parameter for prediction method.

* [Breaking]: Add `training` to C API.

* Change for R and Python custom objective.

* Correct comment.

Co-authored-by: Philip Hyunsu Cho <chohyu01@cs.washington.edu>
Co-authored-by: Jiaming Yuan <jm.yuan@outlook.com>

tests/cpp/test_learner.cc

@@ -159,7 +159,6 @@ TEST(Learner, Json_ModelIO) {
 
   {
     std::unique_ptr<Learner> learner { Learner::Create({p_dmat}) };
-    learner->SetParam("verbosity", "3");
     for (int32_t iter = 0; iter < kIters; ++iter) {
       learner->UpdateOneIter(iter, p_dmat.get());
     }