Remove unused-variable in openr/spark/Spark.cpp

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]]`.

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Reviewed By: palmje

Differential Revision: D65013052

fbshipit-source-id: fb8ef4a8e670792614d30606352ecd908611122f

openr/spark/Spark.cpp

@@ -2022,7 +2022,6 @@ Spark::sendHelloMsg(
   // down event has not arrived yet
   const auto& interfaceEntry = interfaceDb_.at(ifName);
   const auto ifIndex = interfaceEntry.ifIndex;
-  const auto v4Addr = interfaceEntry.v4Network.first;
   const auto v6Addr = interfaceEntry.v6LinkLocalNetwork.first;
   thrift::OpenrVersion openrVer(*kVersion_.version());