Remove unused variables in faiss/IndexIVFPQ.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: palmje

Differential Revision: D53779571

fbshipit-source-id: 008ce210aee7eb8668587cb40154563ada0172db

faiss/IndexIVFPQ.cpp

@@ -1046,7 +1046,7 @@ struct IVFPQScannerT : QueryTables {
             const uint8_t* codes,
             SearchResultType& res) const {
         int ht = ivfpq.polysemous_ht;
-        size_t n_hamming_pass = 0, nup = 0;
+        size_t n_hamming_pass = 0;
 
         int code_size = pq.code_size;