[VPlan] Remove unnecessary DomTreeUpdater flush (NFC).

The current version does not need the explicit flush at this point.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlan.cpp

@@ -1016,8 +1016,6 @@ void VPlan::execute(VPTransformState *State) {
   for (VPBlockBase *Block : RPOT)
     Block->execute(State);
 
-  State->CFG.DTU.flush();
-
   VPBasicBlock *Header = vputils::getFirstLoopHeader(*this, State->VPDT);
   if (!Header)
     return;