s390/pci: remove unused function zpci_rescan()

the only caller of this was removed as part of the suspend/resume
removal so no need to keep this function around.

Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>