Remove Namespaces support in `v3.0`

[Papooch]

Namespaces are a remnant from the early implementation based on cls-hooked, but I've never found an useful application for them, therefore I've decided to remove them to clean up the code base and re-use the API in favor of https://github.com/Papooch/nestjs-cls/issues/30

[Papooch]

Namespaces have been marked as deprecated in #32

[Papooch]

There should still be a guide on how to provide a "namespaced" ClsService in the docs though. One use case would be to store a transaction object there - the lifetime of a transaction can be different to the one of a web request.