removes next_shred_index from return value of entries to shreds api (#21961)

next-shred-index is already readily available from returned data shreds.
The commit simplifies the api for upcoming changes to erasure coding
schema which will require explicit tracking of indices for coding shreds
as well as data shreds.

gossip/src/duplicate_shred.rs

@@ -336,7 +336,7 @@ pub(crate) mod tests {
         })
         .take(5)
         .collect();
-        let (mut data_shreds, _coding_shreds, _last_shred_index) = shredder.entries_to_shreds(
+        let (mut data_shreds, _coding_shreds) = shredder.entries_to_shreds(
             keypair,
             &entries,
             true, // is_last_in_slot