Removed Bookboon links as site now requires signup and no alternative distribution found. (#6166)

books/free-programming-books-subjects.md

@@ -51,7 +51,6 @@ Books that cover a specific programming language can be found in the  [BY PROGRA
 * [atariarchives.org](http://www.atariarchives.org) atariarchives.org makes books, information, and software for Atari and other classic computers available on the Web.
 * [Bento](https://www.bento.io)
 * [Bitsavers.org](http://bitsavers.trailing-edge.com)
-* [Bookboon: IT & Programming](http://bookboon.com/en/it-programming-ebooks) Cookie required. (email address *requested*, not required)
 * [Code Catalog: A Collection of Code Examples from Prominent Open Source Projects](https://codecatalog.org) (HTML)
 * [conceptf1.blogspot.com](http://conceptf1.blogspot.com/2013/11/list-of-freely-available-programming.html)
 * [Free Smalltalk Books, collected by Stéphane Ducasse](http://stephane.ducasse.free.fr/FreeBooks.html)
@@ -603,8 +602,6 @@ Books that cover a specific programming language can be found in the  [BY PROGRA
 * [Programming on Parallel Machines; GPU, Multicore, Clusters and More](http://heather.cs.ucdavis.edu/parprocbook) - Norm Matloff
 Kerridge (PDF) (email address *requested*, not required)
 * [The OpenCL Programming Book](https://us.fixstars.com/products/opencl/book/OpenCLProgrammingBook/contents/)
-* [Using Concurrency and Parallelism Effectively – I](http://bookboon.com/en/using-concurrency-and-parallelism-effectively-i-ebook) - Jon Kerridge (PDF) (email address *requested*, not required)
-* [Using Concurrency and Parallelism Effectively – II](http://bookboon.com/en/using-concurrency-and-parallelism-effectively-ii-ebook) - Jon
 
 
 ### Partial Evaluation