[ticket/12360] Fix duplicated creation of session when logging out the user

In 7d8b578 this line was introduced to fix a bug with style.php.

Later since 1d42d1b a new session is always created after
killing the old one.

This causes a duplicated creation of the session everytime
the user logs itself out.

PHPBB3-12360
Merge remote-tracking branch 'Elsensee/ticket/12360' into develop-ascraeus

* Elsensee/ticket/12360:
  [ticket/12360] Fix duplicated creation of session when logging out the user