s390/qdio: clean up AOB handling

I've stumbled over this too many times now... AOBs are only ever used on
Output Queues. So in qdio_kick_handler(), move the call to their handler
into the Output-only path, and get rid of the convoluted contains_aobs()
helper. No functional change.

While at it, also remove
1. the unused sbal_state->aob field. For processing an async completion,
   upper-layer drivers get their AOB pointer from the CQ buffer.
2. an unused EXPORT for qdio_allocate_aob(). External users would have
   no way of passing an allocated AOB back into qdio.ko anyways...

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>