audiomixer: Remove weird and wrong segment handling

audiomixer: Remove weird and wrong segment handling

There's no reason why audiomixer should override the segment
base of upstream with whatever value it got from a SEEK event,
or even worse... with 0 if there was no SEEK event yet. This
broke synchronization if upstream provided a segment base other
than 0, e.g. when using pad offsets.
Also that this code did things conditional on the element's state
should've been a big warning already that something is just wrong.
If this breaks anything else now, let's fix it properly :)

Also don't do fancy segment position trickery when receiving a
segment event. It's just not correct.