Doc Fix: remove mention of combined mode-related kernel parameters

Looks like you removed the combined_mode quirk (yay!) but didn't update
kernel-parameters.txt...  might confuse people.  Here's a patch to remove
mention of it from the documentation.

Signed-off-by:  Jesse Barnes <jesse.barnes@intel.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>