iio: multiplexer: Remove an unused field in struct mux

In "struct mux", the 'indio_dev' field is unused.
Remove it.

Found with cppcheck, unusedStructMember.
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/7bb04abdc2815caf090a6c9ecab2a51d837792a7.1714556499.git.christophe.jaillet@wanadoo.frSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/multiplexer/iio-mux.c

@@ -30,7 +30,6 @@ struct mux {
 	int cached_state;
 	struct mux_control *control;
 	struct iio_channel *parent;
-	struct iio_dev *indio_dev;
 	struct iio_chan_spec *chan;
 	struct iio_chan_spec_ext_info *ext_info;
 	struct mux_child *child;