Remove n_jobs from parameters sent to _model_hash

[thcrock]

Fr @ecsalomon :I think we could eke out some additional performance by decreasing the n_processes in triage and allocating some cores to n_jobs in the ensemble methods (random forests, extra trees), allowing them to parallelize trees.

This is technically supported now by adding n_jobs to the parameters in the search grid. However, n_cores then becomes a parameter in the model group, which is not desirable. Better support could be either an additional n_processes parameter in the pipeline or ignoring n_jobs in the model parameters when making model groups.