8264050: Remove unused field VM_HeapWalkOperation::_collecting_heap_roots

Reviewed-by: coleenp, tschatzl

Author: shipilev

src/hotspot/share/prims/jvmtiTagMap.cpp

@@ -2376,7 +2376,6 @@ class VM_HeapWalkOperation: public VM_Operation {
   Handle _initial_object;
   GrowableArray<oop>* _visit_stack;                 // the visit stack
 
-  bool _collecting_heap_roots;                      // are we collecting roots
   bool _following_object_refs;                      // are we following object references
 
   bool _reporting_primitive_fields;                 // optional reporting