Remove conflicting statement from docstring

The statement:

> This class should never be instantiated outside of the get_filters_from_token helper function.

is invalid since:

* There is no helper function `get_filters_from_token`
* The class is instantiated in the `compile_filter` helper function

It confused me just now for a while. I only did a `grep -r 'get_filters_from_token' ./*` in the Django project, but I couldn't find a reference to it.