Tuleap uses PDO as its main way to access the DB by default since Tuleap 10.3 (see
story #11146) but a fallback to the mysql_* API was kept has a safety measure.
The last reported issue due to the switch to PDO was
request #11855, it is time to definitively remove the support of the mysql_* API.
This steps is needed for moving to PHP 7.2, the mysql_* API not being supported anymore.