Code cleanup in UsrGrpAdminServlet

* Make logger final and log errors instead of stack traces
* Reorder modifiers to match the JLS
* Move array designators to the type
* Remove unnecessary Boolean literals and control flow jumps
* Remove commented out code
* Use pattern-matching instanceof
* Simplify logic by flattening/inverting negated ifs/unnecessary elses
* Remove declarations for exceptions that cannot be thrown