Remove nb_best on ListCorpusMLClasses

Closes #770 (closed)

Poor man's benchmark:

# master
In [3]: %time for corpus in cli.request('ListCorpus'): cli.request('ListCorpusMLClasses', id=corpus['id'])
(...)
Wall time: 48.7s

# this branch
In [4]: %time for corpus in cli.request('ListCorpus'): cli.request('ListCorpusMLClasses', id=corpus['id'])
Wall time: 10.0s