Cleanup in google-c2p resolver after removing GRPC_EXPERIMENTAL_ENABLE_NEW_PICK_FIRST

[apolcyn]

Related: #8550

Currently, experiment GRPC_EXPERIMENTAL_ENABLE_NEW_PICK_FIRST defaults on.

After we remove this experiment completely, we can remove code paths in the google-c2p resolver which query the metadata server at http://metadata.google.internal/computeMetadata/v1/instance/network-interfaces/0/ipv6s

[arjan-bal]

Removal of the old pickfirst policy is blocked on internal issue b/415354418.