Remove unused private fields

Unused since https://reviews.llvm.org/D91762 and triggering
-Wunused-private-field

```
llvm/lib/Transforms/Instrumentation/DataFlowSanitizer.cpp:365:13: error: private field 'GetArgTLS' is not used [-Werror,-Wunused-private-field]
  Constant *GetArgTLS;
            ^
llvm/lib/Transforms/Instrumentation/DataFlowSanitizer.cpp:366:13: error: private field 'GetRetvalTLS' is not used [-Werror,-Wunused-private-field]
  Constant *GetRetvalTLS;
```

Reviewed By: stephan.yichao.zhao

Differential Revision: https://reviews.llvm.org/D91820