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

In 1d42d1b9817050974c8bc8b91bc34a6c3cfbfef8 this line was introduced to
fix a bug with style.php.
Later since 1d42d1b9817050974c8bc8b91bc34a6c3cfbfef8 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