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: meyering

Differential Revision: D52734585

fbshipit-source-id: be5da3411d2965f6cd2516376598576049c03f09

Author: r-barnes

faiss/utils/sorting.cpp

@@ -544,7 +544,6 @@ void bucket_sort_inplace_parallel(
 
         // in this loop, we write elements collected in the previous round
         // and collect the elements that are overwritten for the next round
-        size_t tot_written = 0;
         int round = 0;
         for (;;) {
 #pragma omp barrier
@@ -554,9 +553,6 @@ void bucket_sort_inplace_parallel(
                 n_to_write += to_write_2.lims.back();
             }
 
-            tot_written += n_to_write;
-            // assert(tot_written <= nval);
-
 #pragma omp master
             {
                 if (verbose >= 1) {