remove invalid note

The comment here maybe copied from routers of token, it's not valid
anymore and the function in the comment can not be found either.
This should be the difference between V2 and V3 API.

Change-Id: Ibd98aa2a2021c6d20e294656e695d0df7d010a8a

keystone/auth/routers.py

@@ -22,10 +22,6 @@ class Routers(wsgi.RoutersBase):
     def append_v3_routers(self, mapper, routers):
         auth_controller = controllers.Auth()
 
-        # NOTE(morganfainberg): For policy enforcement reasons, the
-        # ``validate_token_head`` method is still used for HEAD requests.
-        # The controller method makes the same call as the validate_token
-        # call and lets wsgi.render_response remove the body data.
         self._add_resource(
             mapper, auth_controller,
             path='/auth/tokens',