hw/s390x/ipl: Remove the "iplbext_migration" property

hw/s390x/ipl: Remove the "iplbext_migration" property

Now that the old machine types that used this property have been
removed, we can remove the property and the corresponding code.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Acked-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Message-ID: <20250103144232.520383-6-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>