This virtual method was always set to the x86-specific pc_madt_cpu_entry(),
even in piix4 which is also used in MIPS. The previous changes use
pc_madt_cpu_entry() otherwise, so madt_cpu can be dropped.

Since pc_madt_cpu_entry() is now only used in x86-specific code, the stub
in hw/acpi/acpi-x86-stub can be removed as well.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230908084234.17642-4-shentey@gmail.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>