iio: buffer: remove 'scan_el_attrs' attribute group from buffer struct

This field doesn't seem used. It seems that only 'buffer->attrs' was ever
used to extend sysfs attributes for an IIO buffer.

Moving forward, it may not make sense to keep it. This patch removes the
field and it's initialization code.

Since we want to rework IIO buffer, to be able to add more buffers per IIO
device, we will merge [somehow] the 'buffer' & 'scan_elements' groups, and
we will continue to add the attributes to the 'buffer' group.

Removing it here, will also make the rework here a bit smaller, since
this code will not be present.

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