Remove compile-time interface assertions in the generated code

[bufdev]

var _ PingServiceClient = (*pingServiceClient)(nil) // verify interface implementation

We already get this via NewPingServiceClient - this will fail if pingServiceClient doesn't implement PingServiceClient. If this assertion fails, we have a bug in our generated code.

[akshayjshah]

Yep, makes sense. Think this was a remnant of a time when we were returning a concrete type from NewXYZClient.