Remove GC prologue and epilogue callbacks.

runtime/vm/gc_marker.h

Index: runtime/vm/gc_marker.h
diff --git a/runtime/vm/gc_marker.h b/runtime/vm/gc_marker.h
index ace01c41066b8505edff455a29899e28e0ebf239..7bfa182915b802f7ea8267e2008ce225b911b645 100644
--- a/runtime/vm/gc_marker.h
+++ b/runtime/vm/gc_marker.h
@@ -27,14 +27,13 @@ class GCMarker : public ValueObject {
 
   void MarkObjects(Isolate* isolate,
                    PageSpace* page_space,
-                   bool invoke_api_callbacks,
                    bool collect_code);
 
   intptr_t marked_words() { return marked_bytes_ >> kWordSizeLog2; }
 
  private:
-  void Prologue(Isolate* isolate, bool invoke_api_callbacks);
-  void Epilogue(Isolate* isolate, bool invoke_api_callbacks);
+  void Prologue(Isolate* isolate);
+  void Epilogue(Isolate* isolate);
   void IterateRoots(Isolate* isolate,
                     ObjectPointerVisitor* visitor,
                     intptr_t slice_index,