Remove unreferenced footnotes which causes a build failure on ubuntu 18.04

diff -u -r ../llvm-6.0.0.src/docs/Proposals/VectorizationPlan.rst ./docs/Proposals/VectorizationPlan.rst
--- ../llvm-6.0.0.src/docs/Proposals/VectorizationPlan.rst	2017-11-20 13:01:47.000000000 +0100
+++ ./docs/Proposals/VectorizationPlan.rst	2018-04-30 00:30:45.270760735 +0200
@@ -212,7 +212,7 @@
    Polly [7]_.
 
 3. Loop Vectorizer: the Vectorization Plan aims to upgrade the infrastructure of
-   the Loop Vectorizer and extend it to handle outer loops [8,9]_.
+   the Loop Vectorizer and extend it to handle outer loops [8]_, [9]_.
 
 References
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