Remove SUMMARY.md of the unstable book as its autogenerated

Its being autogenerated now, as of PR #42612. It seems I forgot to remove it.
rust-lang · Jun 18, 2017 · 9f6fb8e · 9f6fb8e
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commit 9f6fb8e
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diff --git a/src/doc/unstable-book/src/SUMMARY.md b/src/doc/unstable-book/src/SUMMARY.md