Remove unused variables in gloo/transport/ibverbs/device.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: D52847976

fbshipit-source-id: 6c797d181b3682613c0ab44689461b0515727306

gloo/transport/ibverbs/device.cc

@@ -25,8 +25,6 @@ namespace gloo {
 namespace transport {
 namespace ibverbs {
 
-static const std::chrono::seconds kTimeoutDefault = std::chrono::seconds(30);
-
 // Scope guard for ibverbs device list.
 class IbvDevices {
  public: