src,bootstrap: remove NodeMainInstance::registry_

All uses of NodeMainInstance::registry_ were removed in
nodejs/node@43d2e24,
so this change removes the variable completely.

Signed-off-by: Darshan Sen <[email protected]>

PR-URL: nodejs/node#43392
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Tobias Nießen <[email protected]>
Reviewed-By: Santiago Gimeno <[email protected]>
Reviewed-By: Joyee Cheung <[email protected]>
Reviewed-By: James M Snell <[email protected]>

Author: RaisinTen

src/node_main_instance.h

@@ -78,7 +78,6 @@ class NodeMainInstance {
                    const std::vector<std::string>& args,
                    const std::vector<std::string>& exec_args);
 
-  static std::unique_ptr<ExternalReferenceRegistry> registry_;
   std::vector<std::string> args_;
   std::vector<std::string> exec_args_;
   std::unique_ptr<ArrayBufferAllocator> array_buffer_allocator_;