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>