Fix GN mac toolchain error.

When you run `gn gen out-gn/Debug` on Mac you get the following:

ERROR at //build/toolchain/mac/BUILD.gn:200:12: Build argument has no effect.
      os = current_os
           ^---------
The variable "os" was set as a build argument
but never appeared in a declare_args() block in any buildfile.

BUG=None
TEST=`gn gen out-gn/Debug` passes without the above error
R=dpranke@chromium.org

Review URL: https://codereview.chromium.org/1064063002

Cr-Commit-Position: refs/heads/master@{#324109}

build/toolchain/mac/BUILD.gn

@@ -195,9 +195,6 @@ template("mac_clang_toolchain") {
 
     toolchain_args() {
       current_os = invoker.toolchain_os
-
-      # TODO(dpranke): os is here for backwards compatibility.
-      os = current_os
     }
   }
 }