platform/chrome: Kconfig: Remove the transitional MFD_CROS_EC config

The MFD_CROS_EC config was a transitional Kconfig option to not break
current defconfigs in the kernel. Now, this is not required anymore
because all the defconfigs have been removed this option and migrated to
enable the CrOS EC parts individually.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Tested-by: Gwendal Grignou <gwendal@chromium.org>