The detect_guest_activity function is no longer used, so lets removed it.

[hverkuil: remove dump_guests() as well as that too is now unused]

Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>