remove unnecessary assertion from subtask_cancel

If the task to be cancelled is yielding or was cancelled earlier, then an event
will already have been set for that task, which is normal and does not indicate
a bug.

Fixes #146

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

Author: dicej

crates/wasmtime/src/runtime/component/concurrent.rs

@@ -2409,7 +2409,10 @@ impl ComponentInstance {
                 // Started, but not yet returned or cancelled; send the
                 // `CANCELLED` event
                 task.cancel_sent = true;
-                assert!(task.event.is_none());
+                // Note that this might overwrite an event that was set earlier
+                // (e.g. `Event::None` if the task is yielding, or
+                // `Event::Cancelled` if it was already cancelled), but that's
+                // okay -- this should superscede the previous state.
                 task.event = Some(Event::Cancelled);
                 if let Some(set) = task.waiting_on.take() {
                     let item = match self.get_mut(set)?.waiting.remove(&guest_task).unwrap() {