s390/pci: implement pcibios_remove_bus

Implement pcibios_remove_bus to free arch specific data when a pci
bus is deregistered. While at it remove a useless kzalloc/kfree
wrapper.

Reviewed-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>