[MRG+2] ENH removed manual code for parallel task batching in forests

Forests now use the threading backend that has such a low overhead that batching tasks together does not bring any measurable performance benefit.

This PR removes the manual batching boilerplate to simplify the code and make it easier to understand and maintain.

Furthermore, batching will soon be implemented in joblib. So even for models that use the multiprocessing backend, manually batching tasks will be useless at some point.