Remove unused variables in faiss/IndexIVFFastScan.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: algoriddle

Differential Revision: D52981034

fbshipit-source-id: 0315bb594a2dfaa059132b5f4e41855a4e15034b

faiss/IndexIVFFastScan.cpp

@@ -858,7 +858,7 @@ void IndexIVFFastScan::search_implem_10(
 
     bool single_LUT = !lookup_table_is_3d();
 
-    size_t ndis = 0, nlist_visited = 0;
+    size_t ndis = 0;
     int qmap1[1];
 
     handler.q_map = qmap1;
@@ -906,7 +906,6 @@ void IndexIVFFastScan::search_implem_10(
                     handler,
                     scaler);
 
-            nlist_visited++;
             ndis++;
         }
     }