std: Remove unstable::global. Unused

src/rt/rust_kernel.cpp

@@ -33,8 +33,7 @@ rust_kernel::rust_kernel(rust_env *env) :
     non_weak_tasks(0),
     at_exit_runner(NULL),
     at_exit_started(false),
-    env(env),
-    global_data(0)
+    env(env)
 {
     // Create the single threaded scheduler that will run on the platform's
     // main thread