Documentation: i2o: remove duplicate documentation

We can remove the i2o documentation because a) the subsystem has been
moved to staging with commit 2cbf7fe2 (i2o: move to staging)
anyhow and b) the here removed files are present in the subsystem
directory again. There, README even has an additional paragraph and the
ioctl docs only differ in whitespaces. Well...

Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>