[MemorySSA] Remove unused declaration optimizeUses (NFC)

The declaration was introduced on Aug 2, 2016 in commit
c43aa5a5b6 without a corresponding
definition.

Note that we do have a definition for
MmeorySSA::OptimizeUses::optimizeUses but not for
MmeorySSA::optimizeUses.

llvm/include/llvm/Analysis/MemorySSA.h

@@ -841,7 +841,6 @@ private:
 
   CachingWalker<AliasAnalysis> *getWalkerImpl();
   void buildMemorySSA(BatchAAResults &BAA);
-  void optimizeUses();
 
   void prepareForMoveTo(MemoryAccess *, BasicBlock *);
   void verifyUseInDefs(MemoryAccess *, MemoryAccess *) const;