[Docs] Remove some WIP X86 documentation I accidentally leaked into r328031.

[Docs] Remove some WIP X86 documentation I accidentally leaked into r328031.

I didn't mean to commit it, but I guess I failed to switch branches or stash it in my local tree.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@328124 91177308-0d34-0410-b5e6-96231b3b80d8