ztimer's machinery depends on figuring out if a timer is currently set
or not. It does that using _is_set(), which is also exposed as
ztimer_is_set().
Now when a timer expired and got taken of the timer queue using
_now_next(), the `next` pointer wasn't unset. This caused _is_set() to
wrongly return `true` for that timer.
Internally in ztimer, this didn't cause breakage, just an unnecessary
iteration of the timer queue by _delete_timer_from_list().
But this also broke the public ztimer_is_set().
This commit fixes the issue by correctly NULLing the timer's `next`
pointer when the timer triggers.
