clearTempTables() broken

I noticed that running civicrm-clear-cache from drush wasn't removing temp tables (such as civicrm_export_temp%) and should be. The drush command runs clearDBCache, which then runs clearTempTables. The problem is that we create a new DAO object in order to get the db name, but the object was returning empty.

Since we already have a DAO object in use, we don't really need to create a new one. We just need to reference it and then retrieve the db name. See the attached patch.

I'm not sure if this bug is limited to when the command triggers the clear cache, or also during other instances, such as upgrades.