Remove unused variable (#5457)

Summary:
This PR removes the unused variable that causes CLANG build to fail.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5457

Differential Revision: D15825027

Pulled By: HaoyuHuang

fbshipit-source-id: 72c847c39ca310560efcbc5938cffa6f31164068

trace_replay/block_cache_tracer_test.cc

@@ -141,7 +141,6 @@ class BlockCacheTracerTest : public testing::Test {
 TEST_F(BlockCacheTracerTest, AtomicWriteBeforeStartTrace) {
   BlockCacheTraceRecord record = GenerateAccessRecord();
   {
-    TraceOptions trace_opt;
     std::unique_ptr<TraceWriter> trace_writer;
     ASSERT_OK(NewFileTraceWriter(env_, env_options_, trace_file_path_,
                                  &trace_writer));