<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def unset(self, key):
 """ Remove a key from the database entirely. """

        self.cursor.execute('delete from kv where key=?', [key])
        if self.revision and self.cursor.rowcount:
            self.cursor.execute(
                'insert into kv_revisions values (?, ?, ?)',
                [key, self.revision, json.dumps('DELETED')])