udev-manager: actually remove the property

udev-manager: actually remove the property

AFAICT there's no reason to keep removed entries in the Hashmap
(although it doesn't break anything either). The only use of this
hashmap is in udev_rule_apply_token_to_event(), which just retrieves the
value using hashmap_get(), hence there's no difference there between an
existing entry with a NULL value and a missing entry.