Drop git-checkout-pr feature

[fregante]

Dropping it because:

• the logic behind it is complex and it depends on the user’s local setup
• git-checkout-pr giving wrong code #5932
• it doesn't work in some situations: git-checkout-pr should also work in closed PR or deleted fork #4701
• GitHub already offers a better solution to it, via gh-cli
• GitHub already fixed the reason why it was added (cross-repo PRs checked out via gh now have push access via successive git commands)

closes #5932
closes #4701

[gengjiawen]

It's so usefully when I debug. Can we add it back and not turn it on by default. Gh not available in most cloud dev env, and you can't use it without a token.

In my use case, I am a Node.js collaborator, run into different compile issues on different platform. I use this every time when debuging.