Remove unused variables in faiss/IndexHNSW.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: meyering

Differential Revision: D52734573

fbshipit-source-id: edf315a4e2e7ddfaf8b879b7f4a3fc3a6c2e607f

faiss/IndexHNSW.cpp

@@ -733,7 +733,6 @@ int search_from_candidates_2(
         int level,
         int nres_in = 0) {
     int nres = nres_in;
-    int ndis = 0;
     for (int i = 0; i < candidates.size(); i++) {
         idx_t v1 = candidates.ids[i];
         FAISS_ASSERT(v1 >= 0);
@@ -756,7 +755,6 @@ int search_from_candidates_2(
             if (vt.visited[v1] == vt.visno + 1) {
                 // nothing to do
             } else {
-                ndis++;
                 float d = qdis(v1);
                 candidates.push(v1, d);