src: remove unused variable in node_crypto

env was unused since the ref'd commit

Ref: https://github.com/nodejs/node/commit/e34ee1d2c978d46f8aee3fedda0f32bc2a04d7d4
PR-URL: https://github.com/nodejs/node/pull/11361


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>