Support database purge option in Debian/Ubuntu packaging prerm script

The debian/prerm script does not support a separate purge action; purge is only called on debian/postrm if the user requests the purge option. Since the postrm script is called after the airtime-uninstall script has been removed from the target system, we cannot use postrm to drop the database in the event of a purge this way. So if the user wants to drop the database from now on, they will have to invoke airtime-uninstall --purge manually, before uninstalling the deb package in the normal way. This is probably safer for users anyway.