hw/virtio/virtio-pci: Remove VirtIOPCIProxy::ignore_backend_features field

hw/virtio/virtio-pci: Remove VirtIOPCIProxy::ignore_backend_features field

The VirtIOPCIProxy::ignore_backend_features boolean was only set
in the hw_compat_2_7[] array, via the 'x-ignore-backend-features=on'
property. We removed all machines using that array, lets remove
that property, simplify by only using the default version.

Reviewed-by: Mark Cave-Ayland <mark.caveayland@nutanix.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20260108033051.777361-27-zhao1.liu@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>