Remove redundant migration blacklisting if possible

[carlio]

Adding to the list of 'do not check these paths' for migrations is done in two places:

https://github.com/PyCQA/pylint-django/blob/master/pylint_django/checkers/migrations.py#L159
https://github.com/PyCQA/pylint-django/blob/master/pylint_django/plugin.py#L21

This should be removed unless there is some reason that it is repeated.