libinput: remove evdev_device::devnode

Struct 'evdev_device' has field 'devnode' which is initialized to NULL,
never assigned, and finally free()'d. Therefore it is useless.

Remove the dead field.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>