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/agent/src/tools/read_file_tool.rs

@@ -275,7 +275,9 @@ impl AgentTool for ReadFileTool {
                 project.set_agent_location(
                     Some(AgentLocation {
                         buffer: buffer.downgrade(),
-                        position: anchor.unwrap_or(text::Anchor::MIN),
+                        position: anchor.unwrap_or_else(|| {
+                            text::Anchor::min_for_buffer(buffer.read(cx).remote_id())
+                        }),
                     }),
                     cx,
                 );