staging:iio:meter: Remove unused macro IIO_DEV_ATTR_CH_OFF

This patch removes the macro IIO_DEV_ATTR_CH_OFF. The macro
IIO_DEV_ATTR_CH_OFF is not required, due to the replace of it by the
direct use of IIO_DEVICE_ATTR in files staging/iio/meter/ade7759.c and
staging/iio/meter/ade7753.c.

Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>