Documentation: Remove duplicate x86 index entry

x86 got added twice to the index via the RST conversion and the MDS
documentation changes. Remove one instance.
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/index.rst

@@ -112,7 +112,6 @@ implementation.
 .. toctree::
    :maxdepth: 2
 
-   x86/index
    sh/index
    x86/index