mmc: sh_mmcif: Remove unused ->get_cd() platform callback

Removing the callback also enables us to remove the sh_mmcif_get_cd()
altogether, as we convert to use mmc_gpio_get_cd() to the same kind of
work.

Cc: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>