mmc: test: remove the shutdown function

Since it's doing nothing for shutdown behavior. And the callback will
be checked firstly in mmc_bus_shutdown().

Signed-off-by: Yue Hu <huyue2@yulong.com>
Link: https://lore.kernel.org/r/20210119051425.305-1-zbestahu@gmail.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>