Remove unused exception parameter from openr/common/MainUtil.cpp

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: D52957772

fbshipit-source-id: bb6fdec9d2d4398db8bf95f9929af7227274f8fc

openr/common/MainUtil.cpp

@@ -26,7 +26,7 @@ waitForFibService(const folly::EventBase& signalHandlerEvb, int port) {
     openr::Fib::createFibClient(evb, client, port);
     try {
       switchState = client->sync_getSwitchRunState();
-    } catch (const std::exception& e) {
+    } catch (const std::exception&) {
     }
     // sleep override
     std::this_thread::sleep_for(std::chrono::seconds(1));