Remove unused variables in faiss/IndexIVF.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: palmje

Differential Revision: D53779599

fbshipit-source-id: 7f872cecdff078681748d18dfdea78e361a9a72f

Author: r-barnes

faiss/IndexIVF.cpp

@@ -668,7 +668,6 @@ void IndexIVF::search_preassigned(
 #pragma omp for schedule(dynamic)
             for (int64_t ij = 0; ij < n * nprobe; ij++) {
                 size_t i = ij / nprobe;
-                size_t j = ij % nprobe;
 
                 scanner->set_query(x + i * d);
                 init_result(local_dis.data(), local_idx.data());