Remove an incorrect assert in wake_sleeping_threads()

Currently there is no guarantee that threads are sleeping
when calling wake_sleeping_threads() because put_threads_to_sleep()
returns without waiting for threads to actually sleep.

Assert can be easily triggered calling put_threads_to_sleep() and
wake_sleeping_threads() in a tight loop.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>