iio-private.h: Remove useless field is_output
iio-private.h: Remove useless field is_output

This field is never used anywhere.

Signed-off-by: Paul Cercueil <a027184a55211cd23e3f3094f1fdc728df5e0500@crapouillou.net>

analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio