s390/pci: remove unused bus_next field from struct zpci_dev

This field was added in commit 44510d6f ("s390/pci: Handling
multifunctions") but is an unused remnant of an earlier version where
the devices on the virtual bus were connected in a linked list instead
of a fixed 256 entry array of pointers.

It is also not used for the list of busses as that is threaded through
struct zpci_bus not through struct zpci_dev.

Reviewed-by: Pierre Morel <pmorel@linux.ibm.com>
Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>