src,bootstrap: remove NodeMainInstance::registry_

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

Signed-off-by: Darshan Sen <raisinten@gmail.com>

PR-URL: #43392
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

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_;