Sort loss configs

[sfluegel05]

BCE

Currently, we have several BCE-related configs. Some need to be updated after #132, some have been introduced in #130.

Goal

Have exactly two configs for BCE:

• An unweighted BCE bce_unweighted.yml
• A weighted BCE bce_weighted.yml that uses the same beta as in the Chebifier paper

If someone wants another BCE variation, I'd say they should write the config file themselves.

@schnamo Are there other BCE configurations (for non-multilabel problems) that you would like to include as "standard configurations"?

Other loss configs

• weighting_chebi100.yml is clearly outdated
• focal_loss_12.yml seems very specific - why do we need a number of classes for multilabel @schnamo? The implementation says that is only needed for multiclass problems.
• The rest can stay as it is imo