media: vimc: sen: remove unused kthread_sen field

The field kthread_sen in the vimc_sen_device is
not set and used. So remove the field and
the code that check if it is non NULL

Signed-off-by: Dafna Hirschfeld <dafna.hirschfeld@collabora.com>
Cc: <stable@vger.kernel.org>      # for v5.4 and up
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>