Remove unused variable distinguish_unstaged_diff_hunks (#25462)

Release Notes:

- N/A

crates/git_ui/src/project_diff.rs

@@ -127,7 +127,6 @@ impl ProjectDiff {
                 window,
                 cx,
             );
-            diff_display_editor.set_distinguish_unstaged_diff_hunks();
             diff_display_editor.set_expand_all_diff_hunks(cx);
             diff_display_editor.register_addon(GitPanelAddon {
                 workspace: workspace.downgrade(),