grpc: delete deprecated dial option `WithBalancerName`

[WithBalancerName](https://github.com/grpc/grpc-go/blob/431ea809a7676e1da8d09c33ae0d31fcba85f1ff/dialoptions.go#L208) has been marked deprecated for years. Deleting this will clean up some code paths, leading to simpler and more maintainable code.