Remove workaround from select_subclasess()

The workaround was for an issue that was fixed in Django 1.7: https://code.djangoproject.com/ticket/16855 / 349c12d3f5 .

CHANGES.rst

@@ -7,6 +7,7 @@ Unreleased
 - Confirm support for `Django 4.0`
 - Add Spanish translation
 - Add French translation
+- Drop Django 1.7 workaround from `select_subclasses()`
 
 4.2.0 (2021-10-11)
 ------------------

model_utils/managers.py

@@ -72,16 +72,10 @@ class InheritanceQuerySetMixin:
                     )
             subclasses = verified_subclasses
 
-        # workaround https://code.djangoproject.com/ticket/16855
-        previous_select_related = self.query.select_related
         if subclasses:
             new_qs = self.select_related(*subclasses)
         else:
             new_qs = self
-        previous_is_dict = isinstance(previous_select_related, dict)
-        new_is_dict = isinstance(new_qs.query.select_related, dict)
-        if previous_is_dict and new_is_dict:
-            new_qs.query.select_related.update(previous_select_related)
         new_qs.subclasses = subclasses
         return new_qs