Remove unused variables in faiss/invlists/OnDiskInvertedLists.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: palmje

Differential Revision: D53779541

fbshipit-source-id: 7184c46a38d126debaebbda40aedcb4e9a048040

Author: r-barnes

faiss/invlists/OnDiskInvertedLists.cpp

@@ -407,7 +407,7 @@ void OnDiskInvertedLists::update_entries(
     FAISS_THROW_IF_NOT(!read_only);
     if (n_entry == 0)
         return;
-    const List& l = lists[list_no];
+    [[maybe_unused]] const List& l = lists[list_no];
     assert(n_entry + offset <= l.size);
     idx_t* ids = const_cast<idx_t*>(get_ids(list_no));
     memcpy(ids + offset, ids_in, sizeof(ids_in[0]) * n_entry);