Filters are not preserved in polymorphic parent admin

[faxioman]

Inheriting from PolymorphicParentModelAdmin class break the django admin "preserve filter" behavior. Removing the "get_preserved_filters" in polymorphic/admin/parentadmin.py solves the problem.
Why is this override needed?

[duf59]

Just came across this, it seems to be from an old fix, but it completely breaks preserved_filters now.

[niltonpimentel02]

I will attempt to reproduce that behavior here and see if removing get_preserved_filters fixes the problem.