src: remove unused env_ field from env.h

Currently the following compiler warnings is generated:
In file included from ../src/env-inl.h:28:
../src/env.h:521:18:
warning: private field 'env_' is not used [-Wunused-private-field]
    Environment* env_;
                 ^
1 warning generated.

This commit removes this unused field.

PR-URL: https://github.com/nodejs/node/pull/25784


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>