x86/PCI: Drop unused xen_register_pirq() gsi_override parameter

All callers of xen_register_pirq() pass -1 (no override) for the
gsi_override parameter.  Remove it and related code.

Link: https://lore.kernel.org/r/20200428153640.76476-1-wei.liu@kernel.org


Signed-off-by: Wei Liu <wei.liu@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>