remove the link to "Operating Systems Concept 9th edition" because it is not a...

remove the link to "Operating Systems Concept 9th edition" because it is not a book, but a bunch of slides (#2674)

free-programming-books.md

@@ -579,7 +579,6 @@
 * [How to Make a Computer Operating System ](https://github.com/SamyPesse/How-to-Make-a-Computer-Operating-System) (:construction: *in process*)
 * [How to write a simple operating system in assembly language](http://mikeos.sourceforge.net/write-your-own-os.html) - Mike Saunders (HTML)
 * [Operating Systems and Middleware](https://gustavus.edu/mcs/max/os-book/) (PDF and LaTeX)
-* [Operating Systems Concept 9th edition](http://codex.cs.yale.edu/avi/os-book/OS9/slide-dir/) - Avi Silberschatz, Peter Baer, Galvin Greg Gagne (PDF)
 * [Operating Systems: Three Easy Pieces](http://pages.cs.wisc.edu/~remzi/OSTEP/) (PDF)
 * [Practical File System Design: The Be File System](http://www.nobius.org/~dbg/practical-file-system-design.pdf) - Dominic Giampaolo (PDF)
 * [Project Oberon: The Design of an Operating System, a Compiler, and a Computer](http://people.inf.ethz.ch/wirth/ProjectOberon/index.html) - Niklaus Wirth & Jürg Gutknecht (PDF)