[ticket/12829] Remove check for pgsql 8.3/8.2

Remove check for pgsql 8.3/8.2 and set `multi_insert` to true.

[PHPBB3-12829](https://tracker.phpbb.com/browse/PHPBB3-12829)
I think that it should be only an assignation in sql_connect because $multi_insert is defined in the upper class