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/collab/src/tests/editor_tests.rs

@@ -1581,7 +1581,10 @@ async fn test_share_project(
     buffer_a.read_with(cx_a, |buffer, _| {
         buffer
             .snapshot()
-            .selections_in_range(text::Anchor::MIN..text::Anchor::MAX, false)
+            .selections_in_range(
+                text::Anchor::min_max_range_for_buffer(buffer.remote_id()),
+                false,
+            )
             .count()
             == 1
     });
@@ -1622,7 +1625,10 @@ async fn test_share_project(
     buffer_a.read_with(cx_a, |buffer, _| {
         buffer
             .snapshot()
-            .selections_in_range(text::Anchor::MIN..text::Anchor::MAX, false)
+            .selections_in_range(
+                text::Anchor::min_max_range_for_buffer(buffer.remote_id()),
+                false,
+            )
             .count()
             == 0
     });