Remove unused variables in fboss/agent/state/Interface.cpp

Summary:
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: peygar

Differential Revision: D55099975

fbshipit-source-id: c4c4d45c215f0bf401a9795121fd75bca9bad6d2

Author: r-barnes

fbpcf/frontend/test/benchmarks/FrontendBenchmark.cpp

@@ -19,7 +19,7 @@
 
 namespace fbpcf::frontend {
 
-const bool unsafe = true;
+// const bool unsafe = true;
 const uint32_t batchSize = 1000;
 
 template <class Game0, class Game1>