Remove unused exception parameter from files inc faiss/tests/test_approx_topk.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: algoriddle

Differential Revision: D51977717

fbshipit-source-id: 47a308ff3570935829531663252409b4e0cfdec4

Author: r-barnes

tests/test_approx_topk.cpp

@@ -99,7 +99,7 @@ void test_approx_topk(
                     k,
                     approxDistances.data(),
                     approxIndices.data());
-        } catch (const faiss::FaissException& ex) {
+        } catch (const faiss::FaissException&) {
             //
             if (verbose) {
                 printf("Skipping the case.\n");