security: groupfinder callback is redundant

With Pyramid 2 style security policies, this is already handled in the security policy.

The groupfinder callback used to be passed to the AuthorizationPolicy, but since the Authentication and AuthorizationPolicy have been moved to a singular class this callback isn't used.

src/sambal/security.py

@@ -67,8 +67,3 @@ def logout(request):
         del request.session["samba.realm"]
 
     return forget(request)
-
-
-def groupfinder(username, request):
-    """Callback that returns a list of group names for the current user."""
-    return []