Remove unused exception parameter from gloo/rendezvous/context.cc

Summary:
`-Wunused-exception-parameter` has identified an unused exception parameter. This diff removes it.

This:
```
try {
    ...
} catch (exception& e) {
    // no use of e
}
```
should instead be written as
```
} catch (exception&) {
```

If the code compiles, this is safe to land.

Reviewed By: dmm-fb

Differential Revision: D52957891

fbshipit-source-id: 1b107cbfea042a6dc3a9195d550110b0a9c72841

Author: r-barnes

gloo/rendezvous/context.cc

@@ -46,7 +46,7 @@ ContextFactory::ContextFactory(std::shared_ptr<::gloo::Context> backingContext)
       GLOO_ENFORCE(
         backingContext_->getPair(i) != nullptr,
         "Missing pair in backing context");
-    } catch(std::out_of_range& e) {
+    } catch(std::out_of_range&) {
       GLOO_THROW("Backing context not fully connected");
     }
   }