n-api: fix compiler warning

private field 'async_context' is not used [-Wunused-private-field]

PR-URL: #21597
Refs: #17887
Reviewed-By: Gabriel Schulhof <gabriel.schulhof@intel.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>

Author: cjihrig

src/node_api.cc

@@ -3883,7 +3883,6 @@ class TsFn: public node::AsyncResource {
   void* finalize_data;
   napi_finalize finalize_cb;
   bool idle_running;
-  napi_async_context async_context;
   napi_threadsafe_function_call_js call_js_cb;
   bool handles_closing;
 };