fix(scheduler): drop the global TASKS entry on exit

The global TASKS BTreeMap got a new TaskHandle on every spawn
but was never cleaned up. This made join()'s
TASKS.contains_key(&id) always return true for any tid that ever
existed, and slowly leaked TaskHandles. exit() already removes the
WAITING_TASKS queue — remove the TASKS entry symmetrically.

Author: stlankes

src/scheduler/mod.rs

@@ -194,6 +194,8 @@ impl PerCoreSchedulerExt for &mut PerCoreScheduler {
 					self.custom_wakeup(task);
 				}
 			}
+
+			TASKS.lock().remove(&current_id);
 		});
 
 		self.reschedule();