The commit 6f6c15ef "xen/pvhvm: Remove
the xen_platform_pci int." makes the x86 version of
xen_pci_platform_unplug static.

Therefore we don't need anymore to define a dummy xen_pci_platform_unplug
for ARM.
Signed-off-by: NJulien Grall <julien.grall@citrix.com>
Signed-off-by: NStefano Stabellini <stefano.stabellini@eu.citrix.com>