Remove broken link from problem sets

[ivanvc]

Remove the Caribbean Online Judge site which is not accessible anymore.

What does this PR do?

Remove resource

For resources

Description

According to the Wayback Machine, the last capture from the Caribbean Online Judge site was from March this year, and it was a 502. The last time it didn't respond with an error was in 2020.

Why is this valuable (or not)?

It doesn't seem like it's a temporary glitch, but the site seems to be gone for good.

How do we know it's really free?

N/A

For book lists, is it a book? For course lists, is it a course? etc.

N/A

Checklist:

• Read our contributing guidelines.
• Search for duplicates.
• Include author(s) and platform where appropriate.
• Put lists in alphabetical order, correct spacing.
• Add needed indications (PDF, access notes, under construction).
• Used an informative name for this pull request.

Follow-up

• Check the status of GitHub Actions and resolve any reported warnings!