title: Git - How to remove untracked folder in submodules
body: <p>I tried checkout and discard this library with git checkout command, but seems it does not work for me.</p>
  
  <p>I tried to remove this folder, but after that git wants me to add this deleted folder to a commit, but I don't want to track it or push to server.</p>
  
  <blockquote>
    <p>Changes not staged for commit:   (use "git add ..." to update
    what will be committed)   (use "git checkout -- ..." to discard
    changes in working directory)   (commit or discard the untracked or
    modified content in submodules)</p>
    
    <p>modified:   Libraries/some_sdk (untracked content)</p>
  </blockquote>