Remove `extern mod foo (name="bar")` syntax, closes #9543

This patch for  #9543 throws an `obsolete syntax` error for `extern mod foo (name="bar")` . 
I was wondering if [this](https://github.com/fhahn/rust/compare/mozilla:master...fhahn:issue9543-remove-extern-mod-foo?expand=1#diff-da9d34ca1d0f6beee2838cf02e07345cR4444) is the correct place to do this?

I think the wording of the error message could probably be improved as well.

If this approach is OK, I'm going to run the whole test suite tomorrow and update the old syntax to the new one.