mmc: core: Drop the unused mmc_power_save|restore_host()

The last user of mmc_power_save|restore_host() APIs is gone, hence let's
drop them. Drop also the corresponding bus_ops callback,
->power_save|restore() as those becomes redundant.

Cc: Tony Lindgren <tony@atomide.com>
Cc: Eyal Reizer <eyalreizer@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>