Remove unused-variable in kineto/libkineto/src/RoctracerLogger.cpp +3

Summary:
LLVM-15 has a warning `-Wunused-variable` which we treat as an error because it's so often diagnostic of a code issue. Unused variables can compromise readability or, worse, performance.

This diff either (a) removes an unused variable and, possibly, it's associated code or (b) qualifies the variable with `[[maybe_unused]]`.

#buildsonlynotests - Builds are sufficient

 - If you approve of this diff, please use the "Accept & Ship" button :-)

Differential Revision: D66330328

fbshipit-source-id: 16b9fd6fec99f0e4b18ac2814183df3541fd5cd0

libkineto/src/RoctracerLogger.cpp

@@ -286,8 +286,6 @@ void RoctracerLogger::activity_callback(
     const char* begin,
     const char* end,
     void* arg) {
-  RoctracerLogger* dis = &singleton();
-
   // Log latest completed correlation id.  Used to ensure we have flushed all
   // data on stop
   std::unique_lock<std::mutex> lock(s_flush.mutex_);