We can simply merge the option hashes now because they are not
conditional anymore. Well, the magic `presence` method does the
conditional logic for us now.
Instead of using the auth method name, let's just not supply username
and password when we don't want to authenticate. The two affected
servers AU and CA don't have credentials set anyway. This is compatible.
The specs needed changing though.
Spree used to give you more options to configure ActionMailer but our
setup is much simpler. We can remove unused code.
The removed option was never used by OFN and defaulted to true. We don't
need a database migration because the value isn't set in the database.
