s390/qdio: remove unused function declarations

commit 50f769df ("[S390] qdio: improve inbound buffer acknowledgement")
introduced these declarations, but noone added the actual code for them.

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>