NH-2958 follow up - There is no need to cache XmlSerializer instance

There is no need to cache XmlSerializer instance, as it's creation is slow only on first creation

On first creation of XmlSerializer object, the serialization assembly is created on the fly, so it is slow, but subsequent creations are fast. Follow up to discussion with all the details in pull request https://github.com/nhibernate/nhibernate-core/pull/250