fix session restart on PHP 5.3

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | #12562
| License       | MIT
| Doc PR        | -

this also removes some useless code
useless because that is exactly how $this->saveHandler->getId() works