grpc: delete deprecated dial option WithBalancerName

[easwars]

WithBalancerName has been marked deprecated for years. Deleting this will clean up some code paths, leading to simpler and more maintainable code.

[easwars]

#5232 deleted the API, but looks like there are still some internal users who are using this API, and we might have to re-add this in some limited fashion.

[easwars]

We have taken care of this internally. Closing this.

[dfawley]

This was rolled back, but is (mostly) ready to go again.

[dfawley]

Looks like my memory was hazy. We did not roll this back. However, there is a lingering reference to WithBalancerName that should be cleaned up here:

grpc-go/service_config.go

Lines 60 to 63 in 6417495

	// LB is the load balancer the service providers recommends. The balancer
	// specified via grpc.WithBalancerName will override this. This is deprecated;
	// lbConfigs is preferred. If lbConfig and LB are both present, lbConfig
	// will be used.