[ConstantHoisting] Remove unused declaration optimizeConstants (NFC)

The function was renamed to runImpl on Jul 2, 2016 in commit
071d8306b0, but the old declaration has
remained since.

llvm/include/llvm/Transforms/Scalar/ConstantHoisting.h

@@ -198,7 +198,6 @@ private:
   // constant GEP base.
   bool emitBaseConstants(GlobalVariable *BaseGV);
   void deleteDeadCastInst() const;
-  bool optimizeConstants(Function &Fn);
 };
 
 } // end namespace llvm