Fix docs build failure caused by i2o removal

The movement of the I2O tree into staging broke the DocBook build.  Rather
than redirect the i2o references into staging, it seems better to just
remove them since this code is on its way out anyway.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>