src: remove superfluous env_string string

It's only used once at startup in a single place so create the string
in place instead of caching it for the lifetime of the isolate.

PR-URL: #9213
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

src/env.h

@@ -94,7 +94,6 @@ namespace node {
   V(encoding_string, "encoding")                                              \
   V(enter_string, "enter")                                                    \
   V(env_pairs_string, "envPairs")                                             \
-  V(env_string, "env")                                                        \
   V(errno_string, "errno")                                                    \
   V(error_string, "error")                                                    \
   V(events_string, "_events")                                                 \

src/node.cc

@@ -3280,7 +3280,7 @@ void SetupProcessObject(Environment* env,
 
   Local<Object> process_env =
       process_env_template->NewInstance(env->context()).ToLocalChecked();
-  process->Set(env->env_string(), process_env);
+  process->Set(FIXED_ONE_BYTE_STRING(env->isolate(), "env"), process_env);
 
   READONLY_PROPERTY(process, "pid", Integer::New(env->isolate(), getpid()));
   READONLY_PROPERTY(process, "features", GetFeatures(env));