xds: remove retained resources logics for non State-of-The-World resources

[YifeiZhuang]

I argue there was never a reason for it.
The perceived value was that the dependent resources would be needed. That is true, but they would already be retained by the components outside of xds client. For example, the xds resolver watches the dependent RDS resources that were referenced in LDS.
-ejona@

follow up on #9444
Brief description document of the motivation: @ejona86 it would be great if you can have a glance on it. https://docs.google.com/document/d/18SYwBEt4vLsfKiJ_XOOo8lgi_HTovjz7VEhmCDkmmF4/edit#heading=h.h2ddie7y1g08

[ejona86]

As silly as it seems, Envoy uses the "state of the world" terminology for all resource types when using the non-delta protocol (which is what we are doing). SotW is an aspect of the RPC method and not the resource type. It's just that some resource types behave differently in SotW from each other...

https://www.envoyproxy.io/docs/envoy/latest/api-docs/xds_protocol

The SotW approach was the original mechanism used by xDS, in which the client must specify all resource names it is interested in with each request, and for LDS and CDS resources, the server must return all resources that the client has subscribed to in each request.

So we should use a different word for this method. I don't know what to name it though. "isFullStateOfTheWorld"? Throw a short javadoc on the method to resolve any source of ambiguity.