[ticket/12480] Only load config/routing.yml from extensions route

PHPBB3-12480
[ticket/12480] Add subfolder/config/routing.yml which should not be found

PHPBB3-12480
[ticket/12480] Remove unused use statements

PHPBB3-12480
Merge pull request #2388 from nickvergessen/ticket/12480

[ticket/12480] Only load config/routing.yml from extensions route

* nickvergessen/ticket/12480:
  [ticket/12480] Remove unused use statements
  [ticket/12480] Add subfolder/config/routing.yml which should not be found
  [ticket/12480] Only load config/routing.yml from extensions route