i2c: Delete outdated client porting guide

The document describing how to port i2c chip drivers from Linux 2.4 to
Linux 2.6 is outdated. As I suspect that most drivers that had to be
ported have already been by now, I do not want to spend time updating
it. Let's just delete it instead.

Signed-off-by: Jean Delvare <khali@linux-fr.org>