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/collab/src/tests/following_tests.rs

@@ -289,7 +289,7 @@ async fn test_basic_following(
                 .get_open_buffer(&(worktree_id, "2.txt").into(), cx)
                 .unwrap()
         });
-        let mut result = MultiBuffer::new(0, Capability::ReadWrite);
+        let mut result = MultiBuffer::new(Capability::ReadWrite);
         result.push_excerpts(
             buffer_a1,
             [ExcerptRange {