replace whitespace-cleanup with delete-trailing-whitespace

whitespace-cleanup seems to do some formatting, tabs-to-whitespace and
vice versa,... which are kinda annoying.

We should be fine for now with just removing trailing whitespaces.

config.org

@@ -77,7 +77,7 @@ The audible bell is annoying AF.
 #+BEGIN_SRC emacs-lisp
 (use-package whitespace
   :ensure nil
-  :hook (before-save-hook . whitespace-cleanup)
+  :hook (before-save-hook . delete-trailing-whitespace)
   ;; if we wanna remove this hook at any time, eval:
   ;; (remove-hook 'before-save-hook #'whitespace-cleanup)
   )