Revert "staging: iio: ak8975: add platform data."

This reverts commit f2f1794835f1d8900d2b15d114c54e70c849809b.

It should not be putting code into the include/input/ directory, and
lots of other people have complained about it.

Cc: Tony SIM <chinyeow.sim.xt@renesas.com>
Cc: Andrew Chew <achew@nvidia.com>
Cc: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>