vm.c (mrb_env_unshare): remove unnecessary mrb_write_barrier() call

Since this function does not modify references from env, we don't need
to put write barrier here.

src/vm.c

@@ -378,7 +378,6 @@ mrb_env_unshare(mrb_state *mrb, struct REnv *e, mrb_bool noraise)
     stack_copy(p, e->stack, len);
     e->stack = p;
     MRB_ENV_CLOSE(e);
-    mrb_write_barrier(mrb, (struct RBasic *)e);
     return TRUE;
   }
   else {