Custom __instancecheck__ no longer needed

Would be needed if calling a classmethod in __new__
but as we dont support reverse_delete_rules on embedded
documents there is no longer the need for it.

Refs #227

mongoengine/base.py

@@ -665,18 +665,6 @@ class TopLevelDocumentMetaclass(DocumentMetaclass):
 
         return new_class
 
-    def __instancecheck__(cls, inst):
-        """Custom instance check for isinstance() as registering delete rules or
-        calling a cls method in __new__ seems to change the cls so vanilla
-        isinstance() fails"""
-        is_instance = super(DocumentMetaclass, cls).__instancecheck__(inst)
-        if hasattr(cls, '_meta') and 'delete_rules' in cls._meta and not is_instance:
-            try:
-                is_instance = get_document(cls.__name__) == get_document(inst.__class__.__name__)
-            except NotRegistered:
-                pass
-        return is_instance
-
     @classmethod
     def _unique_with_indexes(cls, new_class, namespace=""):
         unique_indexes = []