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