PCI: Remove PCI_IRQ_LEGACY

Replace the last references to PCI_IRQ_LEGACY with PCI_IRQ_INTX in pci.h
header file. With this change, PCI_IRQ_LEGACY is unused and we can remove
its definition.

Link: https://lore.kernel.org/r/20240325070944.3600338-29-dlemoal@kernel.org


Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>