media: mantis: remove mantis_vp3028.c/mantis_vp3028.h

Thoses files are unused since commit b3b96144 ("V4L/DVB (13795): [Mantis/Hopper] Code overhaul, add Hopper devices into the PCI ID list")
8 year after, we could remove it.

Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>