mn10300/PCI: Remove unused pci_mem_start

Remove unused 'pci_mem_start' variable.

Signed-off-by: Myron Stowe <myron.stowe@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: David Howells <dhowells@redhat.com>
CC: Koichi Yasutake <yasutake.koichi@jp.panasonic.com>