Drop FormidableBoundFieldCache "hack"

[brunobord]

This bit of code has been implemented for Django 1.8, but it looks harder than just pulling out a line of code.
Since it's a bit tricky but it looks only like dead code, I'll leave it in this issue for later cleanup.

the starting point is here: https://github.com/peopledoc/django-formidable/blob/master/formidable/forms/__init__.py#L22-L35

It has links to:

• https://github.com/peopledoc/django-formidable/blob/master/formidable/forms/fields.py#L129-L147
• https://github.com/peopledoc/django-formidable/blob/master/formidable/forms/fields.py#L164-L165 (and other occurrences of the get_bound_field() methods
• • tests.