CONTRIBUTING.md: no submodules anymore (JuliaLang#47506)

CONTRIBUTING.md

@@ -342,7 +342,6 @@ please remove the `backport-X.Y` tag from the originating pull request for the c
  - Avoid working from the `master` branch of your fork, creating a new branch will make it easier if Julia's `master` changes and you need to update your pull request.
  - Try to [squash](http:https://gitready.com/advanced/2009/02/10/squashing-commits-with-rebase.html) together small commits that make repeated changes to the same section of code so your pull request is easier to review, and Julia's history won't have any broken intermediate commits. A reasonable number of separate well-factored commits is fine, especially for larger changes.
  - If any conflicts arise due to changes in Julia's `master`, prefer updating your pull request branch with `git rebase` versus `git merge` or `git pull`, since the latter will introduce merge commits that clutter the git history with noise that makes your changes more difficult to review.
- - If you see any unrelated changes to submodules like `deps/libuv`, `deps/openlibm`, etc., try running `git submodule update` first.
  - Descriptive commit messages are good.
  - Using `git add -p` or `git add -i` can be useful to avoid accidentally committing unrelated changes.
  - When linking to specific lines of code in discussion of an issue or pull request, hit the `y` key while viewing code on GitHub to reload the page with a URL that includes the specific version that you're viewing. That way any lines of code that you refer to will still make sense in the future, even if the content of the file changes.