doc/mb/asus/{p2b-ls,p3b-f}: Remove reference to non-existing document

The document for northbridge/intel/i440bx doesn't exist and it didn't
exist at the time of introduction of these two mainboard documents. So
replace the reference with just the northbridge name.

This fixes the following Sphinx warning:

  WARNING: unknown document: '../../northbridge/intel/i440bx/index'

Change-Id: Iaa67399f9d0e62d5d54ae08f5ebb8c70073c601f
Signed-off-by: Felix Singer <felixsinger@posteo.net>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/77442
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nicholas Chin <nic.c3.14@gmail.com>

0 files changed, 0 insertions, 0 deletions