Remove unused variables in faiss/impl/index_read.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: D52981028

fbshipit-source-id: dbdf5c56e1d594e534183483c236ed2f08f98857

Author: r-barnes

faiss/impl/index_read.cpp

@@ -398,15 +398,12 @@ static void read_NSG(NSG* nsg, IOReader* f) {
     graph = std::make_shared<nsg::Graph<int>>(N, R);
     std::fill_n(graph->data, N * R, EMPTY_ID);
 
-    int size = 0;
-
     for (int i = 0; i < N; i++) {
         for (int j = 0; j < R + 1; j++) {
             int id;
             READ1(id);
             if (id != EMPTY_ID) {
                 graph->at(i, j) = id;
-                size += 1;
             } else {
                 break;
             }