staging:iio:meter: Drop ADE7759 driver

I announced the intent to drop some of these meter drivers
on the IIO list last cycle. This device is obsolete and not easily
obtained.  No one has come forward with suitable test hardware and
the driver would need a lot of work to move out of staging.

As such I am dropping it.  We can always bring it back again
if a user / tester emerges in the future.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>