[Session] remove invalid workaround in session regenerate

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

The original issue #7380 was just caused because the developer missed to save the session before doing the redirect. That's all. This reverts #8270 and following.