Remove outdated PHP 7 Upgrade Guide [skip ci]

README.md

 * [Mastering Object-Orientated PHP](https://masteringobjectorientedphp.com/) - A book about object-orientated PHP by Brandon Savage.
 * [PHP Cookbook](https://www.oreilly.com/library/view/php-cookbook/9781098121310/) - This cookbook provides code recipes to help you resolve a variety of coding issues.
 * [Modernizing Legacy Applications in PHP](https://leanpub.com/mlaphp) - A book about modernizing legacy PHP applications by Paul M. Jones.
-* [PHP 7 Upgrade Guide](https://leanpub.com/php7) - An ebook covering all of the features and changes in PHP 7 by Colin O'Dell.
 * [Scaling PHP Applications](https://www.scalingphpbook.com) - An ebook about scaling PHP applications by Steve Corona.
 * [Securing PHP: Core Concepts](https://leanpub.com/securingphp-coreconcepts) - A book about common security terms and practices for PHP by Chris Cornutt.
 * [Signaling PHP](https://leanpub.com/signalingphp) - A book about catching PCNTL signals in CLI scripts by Cal Evans.