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/movement.rs

@@ -928,7 +928,7 @@ mod tests {
 
             let buffer = cx.new_model(|cx| Buffer::local("abc\ndefg\nhijkl\nmn", cx));
             let multibuffer = cx.new_model(|cx| {
-                let mut multibuffer = MultiBuffer::new(0, Capability::ReadWrite);
+                let mut multibuffer = MultiBuffer::new(Capability::ReadWrite);
                 multibuffer.push_excerpts(
                     buffer.clone(),
                     [