media: videobuf2: Remove vb2_find_timestamp()

Now that we've transitioned all users to vb2_find_buffer API,
remove the unused vb2_find_timestamp().

Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Acked-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>