Remove dead link : Escape from callback hell

[andys8]

The article was removed. The link is still existing but dead. It should be removed.

Dead link

http://elm-lang.org/blog/escape-from-callback-hell

Issues

Mentioned in issues #666, #550 and #450 .

[process-bot]

Thanks for the pull request! Make sure it satisfies this checklist. My human colleagues will appreciate it!

Here is what to expect next, and if anyone wants to comment, keep these things in mind.

[andys8]

Dead link still there.