ringhash: don't strip address.Attributes and stop using address.Metadata

[menghanl]

ringhash policy currently strips Attributes from add the weight to address.Metadata. The reason is that Attributes are not comparable, and the weight change cannot be detected.

https://github.com/grpc/grpc-go/pull/4741/files#diff-62641f60fa804eaaa0871705672007e07191dcdc95b3f74bba62bd89c2f75100R194

After #3611 is fixed, we should remove the special handling.