grpc: delete deprecated API WithBalancerName()

[easwars]

Summary of changes:

• Deleted the WithBalancerName() API which has been deprecated for ages now.
  • This simplifies code in the ClientConn since we no longer need to handle special cases around balancer name being specified through a dial option
• Test cleanups
  • Use WithDefaultServiceConfig instead of WithBalancerName wherever applicable
  • Use WithTransportCredentials(insecure.NewCredentials()) instead of WithInsecure() in tests already touched by the above change
  • Use defaultTestTimeout wherever applicable
  • Turn setupClient and setupServer functions into test helpers and significantly reduce boiler plate in individual tests in healthcheck_test.go

Fixes #5229

RELEASE NOTES:

• grpc: delete WithBalancerName() API, deprecated over 4 years ago in Add dial option to set balancer #1697

[johanbrandhorst]

Hi! I'm here from the new release notes. I think there is some confusion going on, because the release notes (and this PR) claims that WithBalancerName was deprecated 4 years ago, in #1697. However, it appears it was introduced 4 years ago. It appears to have been deprecated in #2900, which was 3 years ago. Could you perhaps provide a migration guide for users of this API to the release notes?

[menghanl]

For use cases that set the balancer for the client, grpc.WithDefaultServiceConfig should work.

- grpc.WithBalancerName(balancerName)
+ grpc.WithDefaultServiceConfig(fmt.Sprintf(`{"loadBalancingConfig": [{"%s":{}}]}`, balancerName))

Note that there can be problems if the client receives service configs later with different load balancing policy (or without load balancing policy). The newer service config will override the default service config.
This behavior is different from grpc.WithBalancerName, where the load balancing policy in service config will be ignored.
There's also grpc.WithDisableServiceConfig, if you don't want service config at all.