Documentation: asus-laptop.txt references an obsolete Kconfig item

The CONFIG_X86_UP_APIC option no longer exists.  Delete the
reference of it.

Cc: trivial@kernel.org
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>