django 1.11: delete ambiguous arguments in authenticate (#21489)

django 1.11 can call LDAPBackendPasswordLost.authenticate
with a request argument instead of a user
due to introspection.

https://code.djangoproject.com/ticket/28207

src/authentic2/backends/ldap_backend.py

@@ -1175,7 +1175,7 @@ class LDAPBackend(object):
 
 
 class LDAPBackendPasswordLost(LDAPBackend):
-    def authenticate(self, user=None, **kwargs):
+    def authenticate(self, user=None):
         if not user:
             return
         config = self.get_config()