mmc: mmci: Remove custom ios handler

The custom boardfile ios handler isn't used anywhere in the
kernel. Delete it.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20220427125557.1608825-1-linus.walleij@linaro.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>