Remove unused variables in faiss/utils/sorting.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: bunnypak, dmm-fb

Differential Revision: D53011673

fbshipit-source-id: 011204944fdc48627a34e1358833fc525dcc2c28

faiss/utils/sorting.cpp

@@ -791,7 +791,6 @@ void hashtable_int64_to_int64_lookup(
     std::vector<int64_t> hk(n), bucket_no(n);
     int64_t mask = capacity - 1;
     int log2_nbucket = log2_capacity_to_log2_nbucket(log2_capacity);
-    size_t nbucket = (size_t)1 << log2_nbucket;
 
 #pragma omp parallel for
     for (int64_t i = 0; i < n; i++) {