utils: Avoid registering the class in ruaml.yaml

Registering the class properly can only be done in a metaclass, which would complicate the use of abc.ABC for example.

Also, not all classes are required to inherit from Serializable, which means that many classes would not get the tag stability provided by registering the class explicitely. Instead, the dotted name of the classes are used.

Therefore, we avoid registering the class altogether, and will rely on keeping stable class names when backward compatibilty is needed. That can be done by keeping the old name available.

Existing yaml_tag class attributes are removed since they are no longer exploited by ruamel.yaml