Remove some dead code in service.go

ServicePortMap.merge had a giant comment explaining its return value,
but nothing ever used that return value.

ServicePort had an InternalTrafficPolicy() method, but nothing used it
(because it was redundant with InternalPolicyLocal().)

pkg/proxy/types.go

@@ -87,8 +87,6 @@ type ServicePort interface {
 	ExternalPolicyLocal() bool
 	// InternalPolicyLocal returns if a service has only node local endpoints for internal traffic.
 	InternalPolicyLocal() bool
-	// InternalTrafficPolicy returns service InternalTrafficPolicy
-	InternalTrafficPolicy() *v1.ServiceInternalTrafficPolicy
 	// HintsAnnotation returns the value of the v1.DeprecatedAnnotationTopologyAwareHints annotation.
 	HintsAnnotation() string
 	// ExternallyAccessible returns true if the service port is reachable via something