def remove_tiers(self, tiers):
for a in tiers:<EOL><INDENT>self.remove_tier(a, clean=False)<EOL><DEDENT>self.clean_time_slots()<EOL>
Remove multiple tiers, note that this is a lot faster then removing
        them individually because of the delayed cleaning of timeslots.

        :param list tiers: Names of the tier to remove.
        :raises KeyError: If a tier is non existent.