IPMI: Remove SMBus driver info from the docs

Some documentation for the SMBus driver is in the IPMI docs, but that
code is not in the kernel tree at this point.  So remove the docs to
avoid confusion.

Signed-off-by: Corey Minyard <cminyard@mvista.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>