[handbook] Remove unused index commands

They only cost performance and broke build
with latexmk

doc/handbook/0_dumux-handbook.tex

@@ -13,7 +13,6 @@
 \usepackage{graphicx}
 
 \usepackage{listings}
-\usepackage{makeidx}
 \usepackage[square,numbers]{natbib}
 \usepackage{pdflscape}
 \usepackage{rotating}
@@ -98,8 +97,6 @@ Universit\"at Stuttgart, Paffenwaldring 61, D-70569 Stuttgart, Germany}\\
 {\normalsize \texttt{\url{http://dumux.org}}}\\
 }
 
-\makeindex
-
 \begin{document}
 
 \maketitle
@@ -157,5 +154,4 @@ in deeper modifications of underlying \Dumux models, classes, functions, etc.
 
 \bibliographystyle{plainnat}
 \bibliography{dumux-handbook}
-\printindex
 \end{document}