MPX was removed in commit 45fc24e89b7c ("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
Fixes: 45fc24e89b7cc ("x86/mpx: remove MPX from arch/x86")
Acked-by: Dave Hansen
Signed-off-by: Jonathan Corbet