sensors: ak8975: remove passthrough support for MPU9150

Remove the support for enabling passthrough mode support for MPU9150
on the AK8975.  We don't have a proper MPU9150 driver and the MPU9150
has been EOL.  So its highly unlikely this code is being used.

Additonally we remove the device tree binding for the MPU9150 since
we don't have a proper driver for it.

Signed-off-by: Kumar Gala <galak@kernel.org>