sys/event: Remove incorrect comment

The value of `queue->waiter` at the time the event was queued (with IRQs
disabled) was backed up to the stack-variable `waiter`. Thus, the test later on
for `waiter` checks if the queue was already claimed at the time the event
was queued. Therefore, there is no race.

sys/event/event.c

@@ -63,11 +63,6 @@ void event_post(event_queue_t *queue, event_t *event)
     thread_t *waiter = queue->waiter;
     irq_restore(state);
 
-    /* WARNING: there is a minimal chance, that a waiter claims a formerly
-     *          detached queue between the end of the critical section above and
-     *          the block below. In that case, the new waiter will not be woken
-     *          up. This should be fixed at some point once it is safe to call
-     *          thread_flags_set() inside a critical section on all platforms. */
     if (waiter) {
         thread_flags_set(waiter, THREAD_FLAG_EVENT);
     }