media: videobuf2-core: drop obsolete sanity check in __vb2_queue_free()

The sanity check in __vb2_queue_free() is obsolete ever since commit
f035eb4e ("[media] videobuf2: fix lockdep warning"). Remove it and
let __vb2_queue_free() return void.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>