removes invalid links from documentation

doc/src/availability.dox

@@ -11,16 +11,10 @@
 
 \section Tutorial
 
-[YAC Version 1.5.5](https://www.dkrz.de/en-pdfs/en-docs/en-YAC_Tutorial_v155.pdf)
-
 [YAC Version 2.0.0](https://www.dkrz.de/en-pdfs/en-docs/en-YAC_Tutorial_v200.pdf)
 
 [YAC Version 2.4.2](https://www.dkrz.de/pdfs/docs/yac_tutorial_v242.pdf)
 
-\section mailinglist Mailinglist
-
-[https://lists.dkrz.de/mailman/listinfo/yac-user](https://lists.dkrz.de/mailman/listinfo/yac-user)
-
 \section support Support
 
 For support contact the mailing list or any of the authors.