staging: iio: meter: Drop ade7854 driver

This driver is so far from making correct use of the IIO infrastructure
and ABI that if someone wanted to make the driver suitable for moving
out of staging, they would more or less be starting from scratch.

As such there is little point in keeping the existing code in staging.

Note this was only user of the meter.h header so that is dropped.
There are no other drivers in the staging/iio/meter directory so drop
the build system files as well.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Link: https://lore.kernel.org/r/20230129160805.747745-1-jic23@kernel.org