Remove ignored/private packages on "add a changeset if you're a maintainer" link

[wlee221]

Related Issues:

#39: Same root cause, but different use case.
changesets/changesets#436: same issue in CLI.

Problem:

Hi, I've been really enjoying the DX of "add a changeset if you're a maintainer". But whenever a PR modifies both public and ignored/private packages, changeset/bot lists all of them in the yaml header. If we don't remove those ignored/private packages manually, this will throw

Mixed changesets that contain both ignored and not ignored packages are not allowed

Example PR: aws-amplify/amplify-ui#628. This repository has private docs and e2e packages that are ignored, but they still show up in the "add a changeset" link.

Expected Behavior

Remove ignored and/or private packages from changed packages list. This will especially help external contributors that aren't too familiar with the repo.