Fix scrollbar flicker when streaming agent2 response (#36606)

This was caused by calling `list_state.splice` on updated entries. We
don't need to splice the entry, as we'll recompute its measurements
automatically when we render it.

Release Notes:

- N/A

crates/agent_ui/src/acp/thread_view.rs

@@ -793,7 +793,6 @@ impl AcpThreadView {
                 self.entry_view_state.update(cx, |view_state, cx| {
                     view_state.sync_entry(*index, thread, window, cx)
                 });
-                self.list_state.splice(*index..index + 1, 1);
             }
             AcpThreadEvent::EntriesRemoved(range) => {
                 self.entry_view_state