Revert vendor directory to support old golang (GOPATH mode)

gomodule is still in progress to migrate for now, hence multus
team decide to keep vendor directory to support build without
gomodule.

vendor/google.golang.org/grpc/codegen.sh

@@ -0,0 +1,17 @@
+#!/usr/bin/env bash
+
+# This script serves as an example to demonstrate how to generate the gRPC-Go
+# interface and the related messages from .proto file.
+#
+# It assumes the installation of i) Google proto buffer compiler at
+# https://github.com/google/protobuf (after v2.6.1) and ii) the Go codegen
+# plugin at https://github.com/golang/protobuf (after 2015-02-20). If you have
+# not, please install them first.
+#
+# We recommend running this script at $GOPATH/src.
+#
+# If this is not what you need, feel free to make your own scripts. Again, this
+# script is for demonstration purpose.
+#
+proto=$1
+protoc --go_out=plugins=grpc:. $proto