n-api: fix compiler warning

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

PR-URL: https://github.com/nodejs/node/pull/21597
Refs: https://github.com/nodejs/node/pull/17887


Reviewed-By: Gabriel Schulhof <gabriel.schulhof@intel.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>