Fix EZP-22910: Formtoken exception is thrown when logging in without
debug redirection

Regression from [EZP-22767](https://jira.ez.no/browse/EZP-22767).
LegacyHelper is filled even if the result is an ezpKernelRedirect,
triggering a form token exception when request is POST.
Makes sense since LegacyHelper uses runCallback() which in this case
re-triggers request parsing by the legacy kernel. As request is POST, it
legitimately looks for the form token.

For some reason, the issue does not happen with XDebug on.