Removed unused custom exception support for ImmutableList.

If the warning provided was an instance of Exception, then it would be
used as-is. In practice this is untested, unused and ImmutableList is
an undocumented internal datastructure.

django/utils/datastructures.py

@@ -230,11 +230,8 @@ class ImmutableList(tuple):
         self.warning = warning
         return self
 
-    def complain(self, *wargs, **kwargs):
-        if isinstance(self.warning, Exception):
-            raise self.warning
-        else:
-            raise AttributeError(self.warning)
+    def complain(self, *args, **kwargs):
+        raise AttributeError(self.warning)
 
     # All list mutation functions complain.
     __delitem__ = complain