iio: Remove get_bytes_per_datum() from iio_buffer_access_funcs

There haven't been any users of the get_bytes_per_datum() callback for a
while. The core assumes that the number of bytes per datum can be calculated
based on the enabled channels and the storage size of the channel and
iio_compute_scan_bytes() is used to compute this number. So remove the
callback.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>