Fix #349 -- subtract lists in Baker.get_fields() instead of casting to set

[amureki]

No description provided.

[amureki]

@model-bakers/core what do you think of such change?
I was digging the history and found that get_related was added to get related fields except M2M:
berinhard/model_mommy@c5fd18a

However, this functionality did change later, and we are including M2M fields now again - which in the end duplicates Django API:
https://github.com/django/django/blob/f30c7e381c94f41d361877d8a3e90f8cfb391709/django/db/models/options.py#L604-L625

So, I removed it completely along with subtracting lists refactoring to fix #349

[timjklein36]

Looks good!