staging: media: drop vb2_ops_wait_prepare/finish

Since commit 88785982 ("media: vb2: use lock if wait_prepare/finish
are NULL") it is no longer needed to set the wait_prepare/finish
vb2_ops callbacks as long as the lock field in vb2_queue is set.

Since the vb2_ops_wait_prepare/finish callbacks already rely on that field,
we can safely drop these callbacks.

This simplifies the code and this is a step towards the goal of deleting
these callbacks.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org> # for meson/vdec
Tested-by: Luca Ceresoli <luca.ceresoli@bootlin.com> # drivers/staging/media/tegra-video/vi.c