UndertowReactiveWebServerFactory has DeploymentInfoCustomizers even though it does not create a DeploymentInfo

[wilkinsona]

UndertowReactiveWebServerFactory has DeploymentInfoCustomizers but never does anything with them as it does not create a DeploymentInfo. Not creating a DeploymentInfo makes sense as it's a Servlet-specific concept. We should deprecate the methods for adding and getting the customizers and then remove them in due course.