iio: buffer-dma: Remove unused iio_buffer_block struct

This structure was never used anywhere, so it can safely be dropped.

It will later be re-introduced as a different structure in a
different header.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Link: https://lore.kernel.org/r/20211115141925.60164-3-paul@crapouillou.net


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>