samsung: mmc: Drop old MMC init code

Now that these boards use driver model we can drop the old code. At
present s5p_mmc_init() is still used by goni and smdkv310 so cannot be
removed unless we remove those boards.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Minkyu Kang <mk7.kang@samsung.com>