8272884: Make VoidClosure::do_void pure virtual

Reviewed-by: kbarrett, dholmes

src/hotspot/share/memory/iterator.cpp

@@ -40,10 +40,6 @@ void ObjectToOopClosure::do_object(oop obj) {
   obj->oop_iterate(_cl);
 }
 
-void VoidClosure::do_void() {
-  ShouldNotCallThis();
-}
-
 void CodeBlobToOopClosure::do_nmethod(nmethod* nm) {
   nm->oops_do(_cl);
   if (_fix_relocations) {

src/hotspot/share/memory/iterator.hpp

@@ -283,9 +283,7 @@ class MonitorClosure : public StackObj {
 // A closure that is applied without any arguments.
 class VoidClosure : public StackObj {
  public:
-  // I would have liked to declare this a pure virtual, but that breaks
-  // in mysterious ways, for unknown reasons.
-  virtual void do_void();
+  virtual void do_void() = 0;
 };