Don't clear the ThreadGroup when Thread terminates

CRuby does not clear the ThreadGroup set in the Thread when the
thread terminates. We do, which leads to some unpredictability in
specs that check the group against a very short-lived thread. This
patch removes the call to clear the thread's group.

core/src/main/java/org/jruby/RubyThreadGroup.java

@@ -120,7 +120,6 @@ void addDirectly(RubyThread rubyThread) {
 
     public void remove(RubyThread rubyThread) {
         synchronized (rubyThread) {
-            rubyThread.setThreadGroup(null);
             rubyThreadList.remove(rubyThread);
         }
     }