docs: remove MPX from the x86 toc

MPX was removed in commit 45fc24e8 ("x86/mpx: remove MPX from
arch/x86"), this removes the corresponding entry in the x86 toc.

This was suggested by a Sphinx warning.

Signed-off-by: Stephen Kitt <steve@sk2.org>
Fixes: 45fc24e8 ("x86/mpx: remove MPX from arch/x86")
Acked-by: Dave Hansen <dave.hansen@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>