hw/char/serial: remove duplicate .class_init in serial_mm_info

.class_init is already set to serial_mm_class_init.

Remove the duplicate entry.

Fixes: 17fd1a64 ("serial-mm: add "regshift" property")
Cc: marcandre.lureau@redhat.com
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Li Qiang <liq3ea@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20201009113843.60995-1-lvivier@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>