Simplify Arena destructor

Summary: Use the intrusive list's clear_and_dispose API to simplify things somewhat.

Reviewed By: yfeldblum

Differential Revision: D22199152

fbshipit-source-id: fb6ec4e206660bc7628ddd87edba3698d8a2af6d

folly/memory/Arena-inl.h

@@ -87,10 +87,7 @@ void Arena<Alloc>::merge(Arena<Alloc>&& other) {
 
 template <class Alloc>
 Arena<Alloc>::~Arena() {
-  auto disposer = [this](Block* b) { b->deallocate(this->alloc()); };
-  while (!blocks_.empty()) {
-    blocks_.pop_front_and_dispose(disposer);
-  }
+  blocks_.clear_and_dispose([this](Block* b) { b->deallocate(this->alloc()); });
 }
 
 } // namespace folly