Remove the newsletter link

The newsletter link does not work anymore. Also, AFAIK, it was not in active use anyway.

README.md

@@ -78,7 +78,6 @@ just remember that you can't purchase success!
 
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 - You can also interact through GitHub issues. If there is a problem with a course, or a change needs to be made to the curriculum, this is the place to start the conversation. Read more [here](CONTRIBUTING.md).
-- Subscribe to our [newsletter](https://tinyletter.com/OpenSourceSocietyUniversity).
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 > **Warning:** There are a few third-party/deprecated/outdated material that you might find when searching for OSSU. We recommend you to ignore them, and only use the [OSSU CS website](https://cs.ossu.dev) or [OSSU CS Github Repo](https://github.com/ossu/computer-science). Some known outdated materials are: