Remove invalid TODO

We can't drop support for RAUW entirely in `MDNode`s, since it's
required for graph construction.  This comment was from before I'd done
the math on that (out-of-tree), and never should have been committed.

llvm-svn: 225334

include/llvm/IR/Metadata.h

@@ -700,7 +700,6 @@ public:
 /// collision, the uniquing bit is dropped.
 ///
 /// TODO: Make uniquing opt-out (status: mandatory, sometimes dropped).
-/// TODO: Drop support for RAUW.
 class GenericMDNode : public MDNode {
   friend class Metadata;
   friend class MDNode;