ENH removed manual code for parallel task batching 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 removes the 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.
ENH: parallel feature importances for forests