Remove or replace "Why you should never use MongoDB"

[rvedotrc]

https://github.com/HackYourFuture-CPH/databases/blob/main/lesson3/README.md#pre-class-readings

• it's an http link, so generates warnings in modern browsers (the page is not available over https)
• given that we're not trying to teach that MongoDB should never be used, linking to an article with this title is not a great idea
• it's a really long and rambling article, that's clearly not all relevant. We should try to find something more focussed on whatever it is that we are trying to teach.