Removed unnecessary PredictBatch calls

[ShvetsKS]

in #6693
lines were deleted:

if (tree_end - tree_begin == 0) {
      return;
    }

it brings some overheads on training stage:

dataset	#6693	current PR
mortgage1q	20.85s	15.78s
airline-ohe	79.32s	37.028s
higgs1m	18.87s	15.73s
msrank	73.82s	66.7s

[trivialfis]

Sorry, rebasing gone wrong.

[trivialfis]

It was supposed to be moved into GBTree to avoid duplicated code. Somehow lost in the progress. Could you please help moving it into the PredictBatch in GBTree and Dart? I can open a PR too, my bad. ;-(

[ShvetsKS]

It was supposed to be moved into GBTree to avoid duplicated code. Somehow lost in the progress. Could you please help moving it into the PredictBatch in GBTree and Dart? I can open a PR too, my bad. ;-(

no problem. will be moved :)

[ShvetsKS]

@trivialfis seems for Dart it's not a problem as there is no such situation when tree_begin == tree_end
https://github.com/dmlc/xgboost/blob/master/src/gbm/gbtree.cc#L662

For GBTree yes it's moved

[trivialfis]

Great! Let's just put it in GBTree.