Delete session vars after login event is fired

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | --
| Referenced tickets | zikula-modules/Legal/pull/24, #900, #901
| License       | MIT
| Doc PR        | --