staging:iio:accel:sca3000 Drop custom measurement mode attributes

This is now represented by the standard 3db filter frequency controls.
Things get complex wrt to the sampling frequency as these modes change
but that is fine under the IIO ABI where any value is allowed to effect
any other.

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