s390/pci: remove disable_device implementation

pci_disable_device is called by a driver after it stops using the pci
function - e.g. during the removal of the driver. The current
implementation removes the architecture specific information of this
function such that even after a call to pci_enable_device the pci
function is no longer usable. Just remove pcibios_disable_device.

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>