Thank you for looking into this!
Yes, basically what you said :)
just a matter of removing this rule
I think so, it looks like a left-over we forgot to remove, but to be totally sure, you should remove it, build your changes, and run firefox to test that everything works well (docs about getting, building and running the code here: https://docs.firefox-dev.tools/getting-started/build.html)
then uploading this patch as an attachment to Bugzilla for review
That is definitely a possibility and is exactly how we used to do things until recently. So you're welcome to do this.
However the Mozilla project now also uses Phabricator for code reviews. It's much easier to request a review, and much easier for reviewers to look at and merge the code. More info here: https://docs.firefox-dev.tools/contributing/code-reviews.html
I'm assigning the bug to you now.