internal/balancer: Remove a TODO in stub balancer. (#3919)

diff --git a/internal/balancer/stub/stub.go b/internal/balancer/stub/stub.go
index 2ddaf5f..e3757c1 100644
--- a/internal/balancer/stub/stub.go
+++ b/internal/balancer/stub/stub.go
@@ -46,10 +46,6 @@
 }
 
 type bal struct {
-	// TODO: Remove this once the legacy balancer API is removed. See
-	// https://github.com/grpc/grpc-go/pull/3431.
-	balancer.Balancer
-
 	bf BalancerFuncs
 	bd *BalancerData
 }