[Router] Cleanup

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 
Removed unused imports, variables, arguments.
It think it is because PhpStorm does not report dead assignments when the assigned value is ``null``, and so he missed it (``$staticPrefix`` as well btw)