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/diagnostics/src/diagnostic_renderer.rs

@@ -284,7 +284,7 @@ impl DiagnosticBlock {
                     if range.context.overlaps(&diagnostic.range, &snapshot) {
                         Self::jump_to(
                             editor,
-                            Anchor::range_in_buffer(excerpt_id, buffer_id, diagnostic.range),
+                            Anchor::range_in_buffer(excerpt_id, diagnostic.range),
                             window,
                             cx,
                         );

crates/diagnostics/src/diagnostics.rs

@@ -308,7 +308,7 @@ impl ProjectDiagnosticsEditor {
                 .selections
                 .all_anchors(&snapshot)
                 .iter()
-                .filter_map(|anchor| anchor.start.buffer_id)
+                .filter_map(|anchor| anchor.start.text_anchor.buffer_id)
                 .collect::<HashSet<_>>()
         });
         for buffer_id in buffer_ids {