Remove Django < 1.6 'fields' handling

[jonashaag]

In ModelFormSetMixin and BaseInlineFormSetMixin, remove the code that overwrites self.fields if Django >= 1.6 entirely and let Django handle it.

This breaks django-extra-view backwards compatibility, but the introduction of mandatory fields was backwards-incompatible in Django too, and we should rather stick with the new behavior introduced by Django rather than keep Django's old behavior forever. (It was changed in Django for good reasons, namely security.)