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