Undeprecate SubsystemRegistration.registerXMLElementWriter

It's not clear that using a Supplier<XMLElementWriter> is better or even best practice, so we shouldn't push it.

1) The supplier very likely costs more metaspace, since the class being newed up in a method reference gets loaded anyway, plus the cost of the method reference.

2) Sharing the instance between the writer and the parser is more efficient for one that stores state. (Not completely trivial though, as it means the Extension impl must cache it.)

3) For non-legacy versions of the parser, we don't really want a Supplier. A Supplier defers work until the single threaded parsing phase of boot, and we want a lot of work to happen instead in the concurrently executed Extension.initializeParsers call.