Delete a customer unit alias by the unit's key

Deletes an alias of this unit, without naming its customer. Signals still sent with it then resolve to nothing again. The key is looked up across the organization ignoring letter case. When more than one unit carries it in another letter case (legacy duplicates across customers, or case variants on one customer), the exact spelling decides; when that does not single one out either, the answer is 409 CUSTOMER_UNIT_AMBIGUOUS: use the customer’s unit routes, where the key is exact.