Remove replica_id from MultiBuffers (#18141)

This PR removes the `replica_id` field from the `MultiBuffer` struct.

We were only ever referencing this field to pass when constructing a
`MultiBuffer`, and never used it outside of that.

Release Notes:

- N/A

crates/assistant/src/assistant_panel.rs

@@ -2814,9 +2814,8 @@ impl ContextEditor {
         } else {
             // If there are multiple buffers or suggestion groups, create a multibuffer
             let multibuffer = cx.new_model(|cx| {
-                let replica_id = project.read(cx).replica_id();
-                let mut multibuffer = MultiBuffer::new(replica_id, Capability::ReadWrite)
-                    .with_title(resolved_step.title.clone());
+                let mut multibuffer =
+                    MultiBuffer::new(Capability::ReadWrite).with_title(resolved_step.title.clone());
                 for (buffer, groups) in &resolved_step.suggestion_groups {
                     let excerpt_ids = multibuffer.push_excerpts(
                         buffer.clone(),