aging:iio:magnetometer: remove unused header magnet.h

Now all magnetomers are using iio_chan_spec there is no need for
the macros in magnet.h

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Ź!–Zš1x