drivers: disk: Remove use of unused devicetree "label" property

We should avoid use of the label property in devicetrees.  The
'zephyr,sdmmc-disk' compatible node has a 'label' property set
but there isn't any code utilizing this so removing the property
from any devicetrees that have it set (as well as example in docs).

Signed-off-by: Kumar Gala <galak@kernel.org>