mmc: sdhci: Remove ->platform_init() callback as it's no longer used

The commit 1ef5e49e ("mmc: sdhci-of-esdhc: add/remove some quirks
according to vendor version") moved sdhci-of-esdhc away from using the
->platform_init() callback.

As it was the only user of it and that it seems reasonable to believe that
it won't be needed again, let's just remove it.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>