multi_buffer: Remove redundant buffer id field (#43459)

It is easy for us to get the two fields out of sync causing weird
problems, there is no reason to have both here so.

Release Notes:

- N/A *or* Added/Fixed/Improved ...

Co-authored by: Antonio Scandurra <antonio@zed.dev>

crates/sum_tree/src/tree_map.rs

@@ -72,6 +72,12 @@ impl<K: Clone + Ord, V: Clone> TreeMap<K, V> {
         self.0.insert_or_replace(MapEntry { key, value }, ());
     }
 
+    pub fn insert_or_replace(&mut self, key: K, value: V) -> Option<V> {
+        self.0
+            .insert_or_replace(MapEntry { key, value }, ())
+            .map(|it| it.value)
+    }
+
     pub fn extend(&mut self, iter: impl IntoIterator<Item = (K, V)>) {
         let edits: Vec<_> = iter
             .into_iter()