xds: Remove WithBlock option from the fallback test client (#5066)

diff --git a/interop/grpclb_fallback/client_linux.go b/interop/grpclb_fallback/client_linux.go
index b5a16bf..152bb01 100644
--- a/interop/grpclb_fallback/client_linux.go
+++ b/interop/grpclb_fallback/client_linux.go
@@ -99,7 +99,6 @@
 func createTestConn() *grpc.ClientConn {
 	opts := []grpc.DialOption{
 		grpc.WithContextDialer(dialTCPUserTimeout),
-		grpc.WithBlock(),
 	}
 	switch *customCredentialsType {
 	case "tls":