Delete CppGC teardown code (#213)

V8 is doing the teardown now, so there is no need for node to do it.

Author: gahaas

src/env.h

@@ -139,7 +139,6 @@ class NODE_EXTERN_PRIVATE IsolateData : public MemoryRetainer {
       ArrayBufferAllocator* node_allocator = nullptr,
       const EmbedderSnapshotData* embedder_snapshot_data = nullptr,
       std::shared_ptr<PerIsolateOptions> options = nullptr);
-  ~IsolateData();
 
   SET_MEMORY_INFO_NAME(IsolateData)
   SET_SELF_SIZE(IsolateData)