mmc: atmel-mci: Get rid of platform data leftovers

The commit d2c6d518 ("mmc: atmel-mci: move atmel MCI header file")
made sure that there is no in-kernel user of the platform data. But
at the same time it hadn't removed the code around that data structure.
Finish the job here and remove a dead code.

Fixes: d2c6d518

 ("mmc: atmel-mci: move atmel MCI header file")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20240417165708.2965612-2-andriy.shevchenko@linux.intel.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>