n-api: remove unused napi_env member

`wrap_template` is no longer used since we've switched to v8::Private.

PR-URL: https://github.com/nodejs/node/pull/21127


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>