media: staging: media: remove remains of VIDEO_ATOMISP_OV8858

OV8858 files are left unusable since commit 3a81c766 ("media: staging: atomisp: Remove IMX sensor support")
They are uncompilable since they depends on dw9718.c and vcm.c which was removed.

Remove the OV8858 kconfig and files.

Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>