Make Functions unhashable

Remove the incorrect __hash__ function added to the Function class in
46233d2c2a and make the class unhashable. Fix
the one place we were trying to hash it, which is when it appeared in the
deletion_policy of a resource.

Change-Id: I3d47c99470a2404183bccb80269a18623aa79041

heat/engine/function.py

@@ -123,8 +123,7 @@ class Function(object):
             return NotImplemented
         return not eq
 
-    def __hash__(self):
-        return id(self)
+    __hash__ = None
 
 
 def resolve(snippet):

heat/engine/rsrc_defn.py

@@ -85,7 +85,7 @@ class ResourceDefinitionCore(object):
 
         if deletion_policy is not None:
             assert deletion_policy in self.DELETION_POLICIES
-            self._hash ^= hash(deletion_policy)
+            self._hash ^= _hash_data(deletion_policy)
 
         if update_policy is not None:
             assert isinstance(update_policy, (collections.Mapping,