drivers/iio/dac: remove CONFIG_EXPERIMENTAL

This config item has not carried much meaning for a while now and is
almost always enabled by default. As agreed during the Linux kernel
summit, remove it.

CC: Jonathan Cameron <jic23@cam.ac.uk>
CC: Lars-Peter Clausen <lars@metafoo.de>
CC: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
CC: Michael Hennerich <michael.hennerich@analog.com>
CC: Peter Meerwald <pmeerw@pmeerw.net>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>