Update README.md

Author: rwieruch

README.md

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 Showcasing how to remove intervals (or listeners) on unmounted React components. [Read more about it](https://www.robinwieruch.de/react-warning-cant-call-setstate-on-an-unmounted-component).
 
-## [CodeSandbox Demo](https://codesandbox.io/s/github/the-road-to-learn-react/react-interval-setstate-unmounted-component-performance)
-
 ## Installation
 
 * `git clone git@github.com:the-road-to-learn-react/react-interval-setstate-unmounted-component-performance.git`