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/editor/src/git.rs

@@ -195,7 +195,7 @@ mod tests {
         cx.background_executor.run_until_parked();
 
         let multibuffer = cx.new_model(|cx| {
-            let mut multibuffer = MultiBuffer::new(0, ReadWrite);
+            let mut multibuffer = MultiBuffer::new(ReadWrite);
             multibuffer.push_excerpts(
                 buffer_1.clone(),
                 [