Update go.mod and vendor

vendor/google.golang.org/grpc/credentials/internal/syscallconn_appengine.go

@@ -1,30 +0,0 @@
-// +build appengine
-
-/*
- *
- * Copyright 2018 gRPC authors.
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *     http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- *
- */
-
-package internal
-
-import (
-	"net"
-)
-
-// WrapSyscallConn returns newConn on appengine.
-func WrapSyscallConn(rawConn, newConn net.Conn) net.Conn {
-	return newConn
-}