dm: drop unused helper in MTD header

include/mtd.h might be included by files even if CONFIG_DM is not
enabled. In this case, the call to dev_get_uclass_priv() would trigger
a build error. Because this helper has no user, let's drop it off.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>