How do I recreate the XE database in Oracle 11g Express?
You should have dropped the owners (AKA schemas AKA users that created the tables) of the "mess of unused tables."  If you dropped the database, no way to restore it (unless you have a backup).  Just recreate the database.  And in the future you can drop just the non-system users, like to drop the Oracle Sample schema SCOTT:

drop user SCOTT cascade;