Use correct import paths for protobuf packages

The packages `any`, `empty`, and `timestamp` ship with protoc so there
is no need to pull them out of the go package. This simplifies the
generation script and also corrects the import paths for the standard
protobuf types.

Fixes: #2095

tools/grpc/genproto.sh

@@ -1,9 +1,10 @@
-#!/usr/bin/env bash -x
+#!/usr/bin/env bash
 # stolen from prometheus
 #
 # Generate all protobuf bindings.
 # Run from repository root.
 
+set -x
 set -e
 set -u
 
@@ -26,9 +27,7 @@ for pkg in ${INSTALL_PKGS}; do
 done
 
 GOBGP="${PWD}"
-GOPROTO="$(GO111MODULE=on go list -f '{{ .Dir }}' -m github.com/golang/protobuf)"
 
 echo "generating code"
-protoc -I "${GOPROTO}"/ptypes \
-       -I "${GOBGP}"/api \
+protoc -I "${GOBGP}"/api \
        --go_out=plugins=grpc:${GOBGP}/api "${GOBGP}"/api/*.proto