arm/xen: Drop the definition of xen_pci_platform_unplug

arm/xen: Drop the definition of xen_pci_platform_unplug

The commit 6f6c15ef912465b3aaafe709f39bd6026a8b3e72 "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: Julien Grall <julien.grall@citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
(cherry picked from commit 7ed208ef4ef9dbd03cda8a5b5a85cc78f79ef213)
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>