Remove models that are not referenced in tests

This silences one instance of the warning that's being printed in tests
for versions of Django before 2.0:

> RemovedInDjango20Warning: Managers from concrete parents will soon
> qualify as default managers if they appear before any other managers
> in the MRO.

polymorphic/tests/models.py

@@ -237,21 +237,6 @@ class PlainChildModelWithManager(models.Model):
     objects = PlainMyManager()
 
 
-class MgrInheritA(models.Model):
-    mgrA = models.Manager()
-    mgrA2 = models.Manager()
-    field1 = models.CharField(max_length=10)
-
-
-class MgrInheritB(MgrInheritA):
-    mgrB = models.Manager()
-    field2 = models.CharField(max_length=10)
-
-
-class MgrInheritC(ShowFieldTypeAndContent, MgrInheritB):
-    pass
-
-
 class BlogBase(ShowFieldTypeAndContent, PolymorphicModel):
     name = models.CharField(max_length=10)