sys/vtimer: Fix longterm_tick_timer handling.

Before, the longterm_tick_timer had special handling in update_shortterm.
This approach was bad because the longterm_tick_timer's shooting microseconds time
had different semantics like the rest and thus it could end up in a blocking
position in the priority queue at some point in time, although it should
get executed at another point in time.

Made the longterm_tick_timer handling / meaning of its microseconds the same
as the other timers and also removed seconds, because it is now the same
as longertm_tick_timer.absolute.seconds.

See also https://github.com/RIOT-OS/RIOT/pull/2515