staging:iio:dac: Remove dac.h

Now that all drivers have been converted to channel spec, we can finally remove
the dac.h file since it is no longer used.
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/iio/dac/dac.h

-/*
- * dac.h - sysfs attributes associated with DACs
- */
-
-#define IIO_DEV_ATTR_OUT_RAW(_num, _store, _addr)				\
-	IIO_DEVICE_ATTR(out_voltage##_num##_raw, S_IWUSR, NULL, _store, _addr)