Remove unused-but-set variables in fboss/lib/config/PlatformConfigUtils.cpp +5

Summary:
This diff removes a variable that was set, but which was not used.

LLVM-15 has a warning `-Wunused-but-set-variable` which we treat as an error because it's so often diagnostic of a code issue. Unused but set variables often indicate a programming mistake, but can also just be unnecessary cruft that harms readability and performance.

This set of diffs is divided into _simple_ and _complicated_ changes. This is a simple change, so I've verified by hand that it should be low-impact and safe to auto-land.

For the complex changes. Removing this variable will not change how your code works, but the unused variable may indicate your code isn't working the way you thought it was. I've gone through each of these by hand, but mistakes may have slipped through. If you feel the diff needs changes before landing, **please commandeer** and make appropriate changes: there are hundreds of these and responding to them individually is challenging.

For questions/comments, contact r-barnes.

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

Reviewed By: palmje, dmm-fb

Differential Revision: D56946825

fbshipit-source-id: 2bcd089cd19630822928ba43b3d04d4163958f5c

Author: r-barnes

folly/container/test/ForeachBenchmark.cpp

@@ -275,7 +275,7 @@ BENCHMARK(ForLoopFetch, iters) {
 }
 
 BENCHMARK(ForEachKVNoMacroAssign, iters) {
-  int sumKeys = 0;
+  [[maybe_unused]] int sumKeys = 0;
   std::string sumValues;
 
   BENCHMARK_SUSPEND {
@@ -291,7 +291,7 @@ BENCHMARK(ForEachKVNoMacroAssign, iters) {
 }
 
 BENCHMARK(ForEachKVNoMacroNoAssign, iters) {
-  int sumKeys = 0;
+  [[maybe_unused]] int sumKeys = 0;
   std::string sumValues;
 
   BENCHMARK_SUSPEND {

folly/experimental/symbolizer/test/DwarfBenchmark.cpp

@@ -30,7 +30,7 @@ using namespace folly::symbolizer;
 using namespace folly::symbolizer::test;
 
 FOLLY_NOINLINE void lexicalBlockBar() try {
-  size_t unused = 0;
+  [[maybe_unused]] size_t unused = 0;
   unused++;
   inlineB_inlineA_lfind();
 } catch (...) {