[protobuf] Always generate using v2

Turns out we can't build two different protoc-gen-go binaries in the
same checkout, so the only way to atomically move everything over to v2
is by changing the template.

Change-Id: I78f153399a84a0b249360a33bcce52f6a688a191
diff --git a/proto_library.gni b/proto_library.gni
index 70cbc31..b47bfce 100644
--- a/proto_library.gni
+++ b/proto_library.gni
@@ -31,11 +31,6 @@
 #   generate_go (optional, default false)
 #       Generate Go protobuf stubs.
 #
-#   use_golang_protobuf_v2 (optional, default false)
-#       Use google.golang.org/protobuf/cmd/protoc-gen-go instead of
-#       github.com/golang/protobuf/protoc-gen-go to generate Go protobuf. Does
-#       nothing unless generate_go is true.
-#
 #   cc_generator_options (optional)
 #       List of extra flags passed to the protocol compiler.  If you need to
 #       add an EXPORT macro to a protobuf's C++ header, set the
@@ -130,14 +125,6 @@
   } else {
     generate_go = false
   }
-  if (defined(invoker.use_golang_protobuf_v2)) {
-    use_golang_protobuf_v2 = invoker.use_golang_protobuf_v2
-  } else {
-    use_golang_protobuf_v2 = false
-  }
-  if (!generate_go) {
-    not_needed([ "use_golang_protobuf_v2" ])
-  }
   if (defined(invoker.generator_plugin_label)) {
     # Straightforward way to get the name of executable doesn't work because
     # |root_out_dir| and |root_build_dir| may differ in cross-compilation and
@@ -256,10 +243,7 @@
     protoc_path = get_label_info(protoc_label, "root_out_dir") + "/protoc"
 
     if (generate_go) {
-      protoc_gen_go_label = "//third_party/golibs/github.com/golang/protobuf/protoc-gen-go($host_toolchain)"
-      if (use_golang_protobuf_v2) {
-        protoc_gen_go_label = "//third_party/golibs/google.golang.org/protobuf/cmd/protoc-gen-go($host_toolchain)"
-      }
+      protoc_gen_go_label = "//third_party/golibs/google.golang.org/protobuf/cmd/protoc-gen-go($host_toolchain)"
       protoc_gen_go_path =
           get_label_info(protoc_gen_go_label, "root_out_dir") + "/protoc-gen-go"
     }