Remove Box wrap of Cgroup.hire

It's unnecessary.

Signed-off-by: Tim Zhang <tim@hyper.sh>

tests/devices.rs

@@ -17,8 +17,7 @@ fn test_devices_parsing() {
     }
 
     let h = cgroups::hierarchies::auto();
-    let h = Box::new(&*h);
-    let cg = Cgroup::new(h, String::from("test_devices_parsing"));
+    let cg = Cgroup::new(&*h, String::from("test_devices_parsing"));
     {
         let devices: &DevicesController = cg.controller_of().unwrap();