mmc: core: Remove the ->enable|disable() callbacks

These callbacks have been set to deprecated for some time. The last
user (omap_hsmmc) has moved away from using them, which thus enables
us to completely remove them.

Signed-off-by: NeilBrown <neil@brown.name>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>