Remove unused variables in faiss/IndexShardsIVF.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.

 - If you approve of this diff, please use the "Accept & Ship" button :-)

Reviewed By: bunnypak, dmm-fb

Differential Revision: D53011658

fbshipit-source-id: a8ba9102751def7a584167c3f50e4d02e9e6442c

faiss/IndexShardsIVF.cpp

@@ -137,7 +137,6 @@ void IndexShardsIVF::add_with_ids(
     auto fn = [n, ids, x, nshard, d, Iq](int no, Index* index) {
         idx_t i0 = (idx_t)no * n / nshard;
         idx_t i1 = ((idx_t)no + 1) * n / nshard;
-        const float* x0 = x + i0 * d;
         auto index_ivf = dynamic_cast<IndexIVF*>(index);
 
         if (index->verbose) {