remove include/linux/hdsmart.h

include/linux/hdsmart.h is not used by the kernel and should therefore
be removed.

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Cc: "Robert P. J. Day" <rpjday@crashcourse.ca>,
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>