[ticket/12531] Restore default topic link behavior to subsilver2

This reverts changes made from ticket [this PR](https://github.com/phpbb/phpbb/commit/dbd4b5bdbce1e84e6686f8b76efdf4489a8c081f). This is related to the reversion of topic title links pointing back to the first topic always in a post from ticket [12300](https://tracker.phpbb.com/browse/PHPBB3-12300). However, that ticket only addressed prosilver. This ticket addresses subsilver2, which was forgotten.

https://tracker.phpbb.com/browse/PHPBB3-12531
PHPBB3-12531