staging:iio:ade7758 replace unnecessary event line registration.

This was only used for triggering and hence should never have been
done this way in the first place.  Needs to go prior to cleaning
out some of the registration functions.

Untested but identical in form to those that have been.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>