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/inline_assistant.rs

@@ -1145,7 +1145,7 @@ impl InlineAssistant {
 
                 let deleted_lines_editor = cx.new_view(|cx| {
                     let multi_buffer = cx.new_model(|_| {
-                        MultiBuffer::without_headers(0, language::Capability::ReadOnly)
+                        MultiBuffer::without_headers(language::Capability::ReadOnly)
                     });
                     multi_buffer.update(cx, |multi_buffer, cx| {
                         multi_buffer.push_excerpts(