removes first_coding_index from erasure recovery code (#16646)

first_coding_index is the same as the set_index and is so redundant:
https://github.com/solana-labs/solana/blob/37b8587d4/ledger/src/blockstore_meta.rs#L49-L60

core/benches/shredder.rs

@@ -152,9 +152,8 @@ fn bench_shredder_decoding(bencher: &mut Bencher) {
             coding_shreds[..].to_vec(),
             symbol_count,
             symbol_count,
-            0,
-            0,
-            1,
+            0, // first index
+            1, // slot
         )
         .unwrap();
     })