remove links that are not books (#2680)

"Java: A Beginner's Guide" is just the first chapter of a book
"NIO Framework" is a 6 page paper, also not a book

free-programming-books.md

@@ -1345,7 +1345,6 @@
 * [Introduction to Programming in Java](http://introcs.cs.princeton.edu/java/home/) - Robert Sedgewick and Kevin Wayne
 * [Introduction to Programming Using Java](http://math.hws.edu/javanotes/) - David J. Eck
 * [JAAS in Action](https://www.gitbook.com/book/benoit74/jaas-in-action/details)
-* [Java: A Beginner's Guide](http://www.oracle.com/events/global/en/java-outreach/resources/java-a-beginners-guide-1720064.pdf) - Oracle (PDF)
 * [Java Application Development on Linux (2005)](https://ptgmedia.pearsoncmg.com/images/013143697X/downloads/013143697X_book.pdf) - Carl Albing and Michael Schwarz (PDF)
 * [Java Language and Virtual Machine Specifications](https://docs.oracle.com/javase/specs/) - James Gosling, et al
 * [Java Programming for Kids](https://yfain.github.io/Java4Kids/) - Yakov Fain
@@ -1354,7 +1353,6 @@
 * [Microservices for Java Developers](http://www.oreilly.com/programming/free/microservices-for-java-developers.csp) - Christian Posta *(Just fill the fields with any values)*
 * [Modern Java EE Design Patterns](http://www.oreilly.com/programming/free/modern-java-ee-design-patterns.csp) - Markus Eisele *(Just fill the fields with any values)*
 * [Neural Network Development with Java](https://www.packtpub.com/packt/free-ebook/neural-networks-java) - Alan M. F. Souza and Fabio M. Soares *(Just fill the fields with any values)*
-* [NIO Framework](http://nioframework.sourceforge.net/NIO_Paper.pdf) (PDF)
 * [Object-Oriented vs. Functional Programming](http://www.oreilly.com/programming/free/object-oriented-vs-functional-programming.csp) - Richard Warburton *(Just fill the fields with any values)*
 * [OOP - Learn Object Oriented Thinking & Programming](http://pub.bruckner.cz/titles/oop) - Rudolf Pecinovsky
 * [Open Data Structures (in Java)](http://opendatastructures.org/ods-java.pdf) - Pat Morin (PDF)