PCI: Remove unused pci_request_region_exclusive()

pci_request_region_exclusive() was introduced with commit e8de1481
("resource: allow MMIO exclusivity for device drivers") in 2.6.29 which
was released 2008.

It never had an in tree user since then, so after 11 years later let's
remove it.

Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>