staging:iio: sysfs.h remove unused val2 and dead macro.

Vivien pointed out that 99e5dc45
removed the IIO_DEVICE_ATTR_2 macro but didn't clean up the other
macros that used it.

As it turns out, no one is using val2 in tree, so lets scrap that
until it is needed.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Reported-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>