remove broken broken_link checker (too noisy 😞)

[mashehu]

I guess it is time to say good-bye 👋🏻

[ewels]

Can always add it back again if / when gaurav-nelson/github-action-markdown-link-check#121 is implemented.

Could also look into implementing that as a PR. I actually don't think it should be all that difficult to do. Mostly redirecting the output from the npm markdown-link-check package to /dev/null and then giving a nicer output when printing from errors.txt. It's a fairly short entrypoint.sh script in the action.