Remove The Grumpy Programmer's Guide to Building Testable PHP Applications [skip ci]

The book is outdated and no longer available

README.md

 * [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.
-* [The Grumpy Programmer's Guide to Building Testable PHP Applications](https://leanpub.com/grumpy-testing) - A book about building testing PHP applications by Chris Hartjes.
 * [XML Parsing with PHP](https://www.phparch.com/books/xml-parsing-with-php/) - This book covers parsing and validating XML documents, leveraging XPath expressions, and working with namespaces as well as how to create and modify XML files programmatically.
 
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