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: #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>

src/env.h

@@ -517,8 +517,6 @@ class Environment {
     inline AsyncHooks();
     // Keep a list of all Persistent strings used for Provider types.
     v8::Eternal<v8::String> providers_[AsyncWrap::PROVIDERS_LENGTH];
-    // Keep track of the environment copy itself.
-    Environment* env_;
     // Stores the ids of the current execution context stack.
     AliasedBuffer<double, v8::Float64Array> async_ids_stack_;
     // Attached to a Uint32Array that tracks the number of active hooks for