mfd: ds1wm: Remove remaining header file

The driver was removed after kernel 6.2 rendering the header file
unused.

Signed-off-by: Wilken Gottwalt <wilken.gottwalt@posteo.net>
Link: https://lore.kernel.org/r/ZpTbGHb6EX2Oe7ok@monster.localdomain


Signed-off-by: Lee Jones <lee@kernel.org>