media: mantis: remove orphan mantis_core.h

This file has been an orphan ever since commit b3b96144
("V4L/DVB (13795): [Mantis/Hopper] Code overhaul, add Hopper
devices into the PCI ID list"), having no references except
for the orphan removed by commit 519648be ("media: mantis:
remove orphan mantis_core.c")

Fixes: b3b96144 ("V4L/DVB (13795): [Mantis/Hopper] Code overhaul, add Hopper devices into the PCI ID list")
Link: https://patchwork.linuxtv.org/project/linux-media/patch/1277054487-14384-1-git-send-email-bjorn@mork.no/


Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>