auxdisplay: ht16k33: remove superfluous header files

ht16k33.c doesn't use any macro or function declared in
linux/slab.h. Thus, these files can be removed from ht16k33.c
safely without affecting the compilation.

Signed-off-by: Mianhan Liu <liumh1@shanghaitech.edu.cn>
Acked-by: Robin van der Gracht <robin@protonic.nl>
[reworded]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>