[R] Use new predict function. (#6819)

* Call new C prediction API.
* Add `strict_shape`.
* Add `iterationrange`.
* Update document.

src/tree/updater_quantile_hist.cc

@@ -587,7 +587,6 @@ void QuantileHistMaker::Builder<GradientSumT>::InitSampling(const DMatrix& fmat,
 
 #if XGBOOST_CUSTOMIZE_GLOBAL_PRNG
   std::bernoulli_distribution coin_flip(param_.subsample);
-  size_t used = 0, unused = 0;
   for (size_t i = 0; i < info.num_row_; ++i) {
     if (!(gpair_ref[i].GetHess() >= 0.0f && coin_flip(rnd)) || gpair_ref[i].GetGrad() == 0.0f) {
       gpair_ref[i] = GradientPair(0);