Remove unused variables in gloo/transport/tcp/pair.cc

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: danzimm, meyering

Differential Revision: D52848021

fbshipit-source-id: f1e6f1dcf35666ebf3ed45c00602ecea8f0d6b83

gloo/transport/tcp/pair.cc

@@ -43,7 +43,6 @@ namespace {
 // It is hard coded because making buffers larger than this would not
 // have much impact. Also see socket(7).
 constexpr size_t kMaxSendBufferSize = 32 * 1024 * 1024;
-constexpr size_t kMaxRecvBufferSize = 32 * 1024 * 1024;
 
 } // namespace