Remove unused variables in dynolog/src/ScubaLogger.cpp (#213)

Summary:
Pull Request resolved: #213

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 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]]`, mostly in cases where the variable _is_ used, but, eg, in an `assert` statement that isn't present in production code.

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Reviewed By: danzimm, meyering

Differential Revision: D52848049

fbshipit-source-id: d1d62f0c35d864f7209a3c42b7bb94344250366a

dynolog/src/ScubaLogger.cpp

@@ -17,8 +17,10 @@
 #include <chrono>
 
 namespace dynolog {
-constexpr int HOSTNAME_MAX = 50;
+#ifdef USE_GRAPH_ENDPOINT
 constexpr char kScubaUrl[] = "http://graph.facebook.com/v2.2/scribe_logs";
+#endif
+
 DEFINE_string(
     scribe_category,
     "perfpipe_fair_cluster_gpu_stats",