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.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines