Remove unused variables in faiss/IndexIVFFastScan.cpp (#3439)

Summary:
Pull Request resolved: #3439

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, junjieqi

Differential Revision: D57344013

fbshipit-source-id: adf410139d2e6ca69a26ccdbff8511c9b7620489

faiss/IndexIVFFastScan.cpp

@@ -914,10 +914,6 @@ void IndexIVFFastScan::search_implem_10(
         size_t* nlist_out,
         const NormTableScaler* scaler,
         const IVFSearchParameters* params) const {
-    const size_t max_codes = params ? params->max_codes : this->max_codes;
-    const SearchParameters* quantizer_params =
-            params ? params->quantizer_params : nullptr;
-
     size_t dim12 = ksub * M2;
     AlignedTable<uint8_t> dis_tables;
     AlignedTable<uint16_t> biases;

tutorial/cpp/6-HNSW.cpp

@@ -37,7 +37,6 @@ int main() {
         xq[d * i] += i / 1000.;
     }
 
-    int nlist = 100;
     int k = 4;
 
     faiss::IndexHNSWFlat index(d, 32);