IndexStatistics::loss is local-only — the LanceDB Cloud server never returned it. RemoteTable::index_stats always sets loss: None (see rust/lancedb/src/remote/table.rs mock test). It's vestigial; remove it.

• Remove loss from IndexStatistics and IndexStatisticsImpl in rust/lancedb/src/index.rs.
• Remove the summing logic in NativeTable::index_stats in rust/lancedb/src/table.rs.
• Drop loss from Python and Node.js bindings.
• Note in CHANGELOG: breaking for direct Rust users; remote clients already saw None.