Remove unused variables in faiss/impl/HNSW.cpp

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: algoriddle

Differential Revision: D52981042

fbshipit-source-id: 6748dc957a26a6b5dec3922505c18a1996e3a002

faiss/impl/HNSW.cpp

@@ -851,13 +851,11 @@ HNSWStats HNSW::search(
                 top_candidates.pop();
             }
 
-            int nres = 0;
             while (!top_candidates.empty()) {
                 float d;
                 storage_idx_t label;
                 std::tie(d, label) = top_candidates.top();
                 res.add_result(d, label);
-                nres++;
                 top_candidates.pop();
             }
         }