s390/qdio: remove unused sync-after-IRQ infrastructure

The queue processing is fully decoupled from any preceding interrupt,
so we're no longer making any use of the sync-after-IRQ HW capabilities.

And as SIGA-sync is a legacy feature, there's also not much point in
re-designing the driver & qdio-layer code just so that we can
potentially avoid a few syncs. So just remove all the leftover code.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Reviewed-by: Benjamin Block <bblock@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>