[en] Removing duplicated link on Parallel Programming section

free-programming-books.md

@@ -447,7 +447,6 @@
 * [The OpenCL Programming Book](http://www.fixstars.com/en/opencl/book/OpenCLProgrammingBook/contents/)
 * [Introduction to High-Performance Scientific Computing](http://tacc-web.austin.utexas.edu/veijkhout/public_html/istc/istc.html) - Victor Eijkhout
 * [Introduction to Parallel Computing](https://computing.llnl.gov/tutorials/parallel_comp/) - Blaise Barney
-* [Is Parallel Programming Hard, And, If So, What Can You Do About It?](http://kernel.org/pub/linux/kernel/people/paulmck/perfbook/perfbook.html)
 * [Programming on Parallel Machines; GPU, Multicore, Clusters and More](http://heather.cs.ucdavis.edu/parprocbook) - Norm Matloff
 * [Is Parallel Programming Hard, And, If So, What Can You Do About It?](https://www.kernel.org/pub/linux/kernel/people/paulmck/perfbook/perfbook.html) - Paul E. McKenney