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/editor/src/display_map/block_map.rs

@@ -2976,7 +2976,7 @@ mod tests {
         );
     }
 
-    #[gpui::test(iterations = 100)]
+    #[gpui::test(iterations = 60)]
     fn test_random_blocks(cx: &mut gpui::TestAppContext, mut rng: StdRng) {
         cx.update(init_test);