Get rid of $db->sql_return_on_error(true) trickery when splitting/merging topics

Background: https://github.com/phpbb/phpbb3/pull/988

Instead of potentially doing the update that may fail, add conditions to only update rows that can be updated. The subsequent delete will take care of any left over rows.

My expectation is that the current code does not update any rows if any of them fail on postgres and updates all rows other than failing ones on mysql.

3.0 and 3.1 will need separate patches because of https://github.com/phpbb/phpbb3/pull/988.