kernel: poll: outdated check for expired timeout

[pabigot]

signal_poll_event in kernel/poll.c invokes _is_thread_timeout_expired, which checks whether a timeout dticks field has the flag value _EXPIRED. After the timer updates in 987c0e5 the only reference to that value is in benchmarks/timing_info/src/msg_passing_bench.c.

Since there's no way the test could succeed in any other condition some cleanup needs to be done.

[nashif]

1c4177d changed signal_poll_event and removed _is_thread_timeout_expired, so this issue is not valid anymore.