Remove old references to cpu_arch, os from GN files.

build/toolchain/gcc_toolchain.gni

Index: build/toolchain/gcc_toolchain.gni
diff --git a/build/toolchain/gcc_toolchain.gni b/build/toolchain/gcc_toolchain.gni
index 5f49f683e1fcabe9166abfa4b47c9e1db676c163..0192faae7dec03e1e6b7308ebc325cfdeaaf02de 100644
--- a/build/toolchain/gcc_toolchain.gni
+++ b/build/toolchain/gcc_toolchain.gni
@@ -211,12 +211,6 @@ template("gcc_toolchain") {
       target_os = target_os
       target_cpu = target_cpu
 
-      # TODO(dpranke): These values are here for backwards compatibility and
-      # should be deleted when all of the builders and configs have been
-      # updated.
-      cpu_arch = current_cpu
-      os = current_os
-
       if (defined(invoker.is_clang)) {
         is_clang = invoker.is_clang
       }