cpu/hotplug: Remove unused cpuhp_state CPUHP_PCI_XGENE_DEAD

Now that the XGene MSI driver has been mostly rewritten and doesn't
use the CPU hotplug infrastructure, CPUHP_PCI_XGENE_DEAD is unused.

Remove it to reduce the size of cpuhp_hp_states[].

Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://lore.kernel.org/r/20250708173404.1278635-14-maz@kernel.org

Author: Marc Zyngier

include/linux/cpuhotplug.h

@@ -90,7 +90,6 @@ enum cpuhp_state {
 	CPUHP_RADIX_DEAD,
 	CPUHP_PAGE_ALLOC,
 	CPUHP_NET_DEV_DEAD,
-	CPUHP_PCI_XGENE_DEAD,
 	CPUHP_IOMMU_IOVA_DEAD,
 	CPUHP_AP_ARM_CACHE_B15_RAC_DEAD,
 	CPUHP_PADATA_DEAD,