Remove old reference to user handbook

The user and developer handbooks have been merged since 2019:
80974e04

src/docs/asciidoc/chapter/Java_API/01_introduction.adoc

@@ -4,9 +4,7 @@
 
 For developers who want to build specialized tools on top of ProB, we
 have prepared comprehensive documentation of the ProB Java API together
-with a template that enables a quick start. The documentation is
-available in https://www3.hhu.de/stups/handbook/prob2/prob_user_manual.html[the user manual],
-the template is located at https://github.com/bendisposto/prob2_tooling_template.
+with https://github.com/bendisposto/prob2_tooling_template[a template] that enables a quick start.
 
 == Acknowledgements