iio: buffer: drop left-over 'stufftoread' field

This seems like a left-over from a7348347 ("staging:iio: Add polling
of events on the ring access chrdev.").

Then it was moved into the sca3000 driver around 9dd4694d ("iio:
staging: sca3000: hide stufftoread logic"), and that one seemed to be the
only user of this.

Then it eventually was no longer used after 152a6a88
("staging:iio:accel:sca3000 move to hybrid hard / soft buffer design.")

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>