Drop the notion that a DOMMountPoint automatically has a SchemaContext

We currently require each DOMMountPoint to effectively have a SchemaContext to be able to funnel data and requests as such.

This is not as straightforward, as we want to have mount points which have no schema at all and instead just shuffle raw XMLs (or similar).

DOMMountPoint ends up being a service meeting point, where the services provided may or may not include a DOMSchemaService. They also may or may not include specialty services.