[ORC] Fix -Wunused-variable in LLJIT.cpp (NFC)

llvm-project/llvm/lib/ExecutionEngine/Orc/LLJIT.cpp:684:8:
error: unused variable 'ConcurrentCompilationSettingDefaulted' [-Werror,-Wunused-variable]
  bool ConcurrentCompilationSettingDefaulted = !SupportConcurrentCompilation;
       ^
1 error generated.