s390/pci: move zpci_remove_device() to bus code

The zpci_remove_device() function removes the device from the PCI common
code core which is an operation dealing primarily with the zbus and PCI
bus code. With that and to match an upcoming refactoring of the
symmetric scanning part move it to the bus code.

Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>