mmc: sdhci-pltfm: Remove sdhci_pltfm_unregister()

Now that sdhci_pltfm_unregister() is unused, remove it.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20230811130351.7038-16-adrian.hunter@intel.com


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