doc(kernel): making running_task non-null is not desired anymore

Author: yvt

src/constance/src/kernel.rs

@@ -798,7 +798,6 @@ pub struct State<
     TaskPriority: 'static = <System as KernelCfg1>::TaskPriority,
     TimeoutHeap: 'static = <System as KernelCfg2>::TimeoutHeap,
 > {
-    // TODO: Make `running_task` non-null to simplify runtime code
     /// The currently or recently running task. Can be in a Running or Waiting
     /// state.
     running_task: AtomicRef<'static, TaskCb<System, PortTaskState, TaskPriority>>,