Bug 1591538 - Don't include mozwebidlcodegen in SM tarball; r=sfink

This was done in bug 956597 where including its files won over
massaging the imports to not be reached from the tarball's
build system.

There are now only three references to mozwebidlcodegen in the
tarball:

* /python/mozbuild/mozbuild/frontend/data.py in which it is
  imported from a method on WebIDLCollection (there are no
  WebIDL collections in the tarball);

* /python/mozbuild/mozbuild/frontend/emitter.py to emit
  WebIDL collections;

* /python/mozbuild/mozbuild/backend/common.py in which it was
  made local to the single function using it in bug 1272976.

I don't think it is necessary to include it anymore, but given
it's only 56KB I'm not sure it matters either way.

Differential Revision: https://phabricator.services.mozilla.com/D50738

js/src/make-source-package.sh

@@ -123,8 +123,6 @@ case $cmd in
 + /third_party/python/**
 + /third_party/rust/**
 
-+ /dom/bindings/mozwebidlcodegen/**
-
 + /layout/tools/reftest/reftest/**
 
 + /testing/mozbase/**