[polly] Remove unused variable in ScopBuilder.cpp (NFC)

/llvm-project/polly/lib/Analysis/ScopBuilder.cpp:2725:18:
error: unused variable 'DT' [-Werror,-Wunused-variable]
  DominatorTree *DT = Stmt.getParent()->getDT();
                 ^
1 error generated.

Author: DamonFool

polly/lib/Analysis/ScopBuilder.cpp

@@ -2722,7 +2722,6 @@ void ScopBuilder::checkForReductions(ScopStmt &Stmt) {
   // instruction not in that chain.
   using MemAccPair = std::pair<MemoryAccess *, MemoryAccess *>;
   DenseMap<MemAccPair, MemoryAccess::ReductionType> ValidCandidates;
-  DominatorTree *DT = Stmt.getParent()->getDT();
 
   // Iterate over all write memory accesses and check the loads flowing into
   // it for reduction candidate pairs.