hw/s390x: Remove the obsolete hpage_1m_allowed switch

hw/s390x: Remove the obsolete hpage_1m_allowed switch

The s390-ccw-virtio-3.0 machine was the last one that used the
hpage_1m_allowed switch. Since we removed this machine type, we
can now remove the switch and the related code, too. This allows
us to get rid of the get_machine_class() hack and the big fat
warning comment there.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20250506062148.306084-8-thuth@redhat.com>
Reviewed-by: Eric Farman <farman@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>