[VPlan] Remove stale comment from optimizeInductions (NFC).

As suggested post-commit for a002271, remove the stale comment,
SetVector is no longer used here.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp

@@ -528,8 +528,7 @@ void VPlanTransforms::optimizeInductions(VPlan &Plan, ScalarEvolution &SE) {
         Plan, ID, SE, WideIV->getTruncInst(), WideIV->getPHINode()->getType(),
         WideIV->getStartValue(), WideIV->getStepValue());
 
-    // Update scalar users of IV to use Step instead. Use SetVector to ensure
-    // the list of users doesn't contain duplicates.
+    // Update scalar users of IV to use Step instead.
     WideIV->replaceUsesWithIf(
         Steps, [HasOnlyVectorVFs, WideIV](VPUser &U, unsigned) {
           return !HasOnlyVectorVFs || U.usesScalars(WideIV);