[polly] Remove unused variable 'VectorLoops' in IslNodeBuilder.cpp (NFC)

/home/jiefu/llvm-project/polly/lib/CodeGen/IslNodeBuilder.cpp:80:11: error: unused variable 'VectorLoops' [-Werror,-Wunused-variable]
STATISTIC(VectorLoops, "Number of generated vector for-loops");
          ^
1 error generated.