[Session] remove invalid workaround in session regenerate

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.
bug #12326 [Session] remove invalid hack in session regenerate (Tobion)

This PR was merged into the 2.3 branch.

Discussion
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[Session] remove invalid hack 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. Such mistakes won't happen anymore with #12341

This reverts #8270 and following. Also it makes absolutely no sense to do this only for the `files` save handler which creates huge inconsistencies. All save handlers are affected and it's more a documentation thing.

Commits
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703d906 [Session] remove invalid workaround in session regenerate