acpi_piix4: Remove PCI_RMV_BASE write code

Clarify this register as read-only and remove write code.  No
change in existing behavior.
Signed-off-by: NAlex Williamson <alex.williamson@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

docs/specs/acpi_pci_hotplug.txt

@@ -34,4 +34,4 @@ PCI removability status (IO port 0xae0c-0xae0f, 4-byte access):
 -----------------------------------------------
 
 Used by ACPI BIOS _RMV method to indicate removability status to OS. One
-bit per slot.
+bit per slot.  Read-only

hw/acpi_piix4.c

@@ -531,11 +531,6 @@ static uint32_t pcirmv_read(void *opaque, uint32_t addr)
     return s->pci0_hotplug_enable;
 }
 
-static void pcirmv_write(void *opaque, uint32_t addr, uint32_t val)
-{
-    return;
-}
-
 static int piix4_device_hotplug(DeviceState *qdev, PCIDevice *dev,
                                 PCIHotplugState state);
 
@@ -552,7 +547,6 @@ static void piix4_acpi_system_hot_add_init(PCIBus *bus, PIIX4PMState *s)
     register_ioport_write(PCI_EJ_BASE, 4, 4, pciej_write, s);
     register_ioport_read(PCI_EJ_BASE, 4, 4,  pciej_read, s);
 
-    register_ioport_write(PCI_RMV_BASE, 4, 4, pcirmv_write, s);
     register_ioport_read(PCI_RMV_BASE, 4, 4,  pcirmv_read, s);
 
     pci_bus_hotplug(bus, piix4_device_hotplug, &s->dev.qdev);