When an ApiMapping resource is deployed using the Domain defined in the
DomainName resource, the DomainName resource must be deployed before the
ApiMapping resource.

Since the current logic uses the CloudFormation Output of DomainName as
a fall back, preferring the user provided string first, this dependency
is not expressed in the resulting template.

Remove the preference for the user provided string, will inform
synthesis that the dependency must be declared.

fixes #15464