exim: remove otrs_mail_from_hosts

Remove the hardcoded otrs_mail_from_hosts and add wikimedia_domains to
local_domains instead. This ensures that frontend relays and OTRS are
in-sync with what domains they should expect and removes a hardcoded
domain list for the mail relays.

Change-Id: I8dba1b22b9accb81532b30d159832d51fa9e624b