Adjust scrollbar settings to be expandable (#2507)

Note that this PR cannot be cherrypicked into preview, as it relies on
the new settings infrastructure.

Switches settings from `show_scrollbars: "auto"` to `scrollbar: {show:
"auto", git_diffs: true}`.

fixes
https://linear.app/zed-industries/issue/Z-1650/scroll-bar-feature-settings

Release Notes:

- Changed scrollbar settings from `show_scrollbars: "auto"` to
`scrollbar: {show: "auto", git_diffs: true}`. (preview only)

crates/editor/src/editor.rs

@@ -533,12 +533,6 @@ pub struct EditorSnapshot {
     ongoing_scroll: OngoingScroll,
 }
 
-impl EditorSnapshot {
-    fn has_scrollbar_info(&self, is_singleton: bool) -> bool {
-        is_singleton && self.buffer_snapshot.has_git_diffs()
-    }
-}
-
 #[derive(Clone, Debug)]
 struct SelectionHistoryEntry {
     selections: Arc<[Selection<Anchor>]>,