staging:iio: Remove ad5930/ad9850/ad9852/ad9910/ad9951 dummy drivers

All what these 'drivers' do is expose a single (non standard ABI) sysfs
attribute that when written to does a direct pass-through to spi_write(). This
is rather ugly and does not justify the existence of a driver as the same can
easily done by using the spidev interface.

The drivers will eventually be rewritten as proper IIO ABI compliant drivers
which do have the proper abstraction layers between userspace and the device.
But in the meantime these driver do not add any extra value and just clutter up
the staging area. So just remove them.

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