Remove concept of git diff refresh from Item trait

crates/diagnostics/src/diagnostics.rs

@@ -609,15 +609,6 @@ impl Item for ProjectDiagnosticsEditor {
         unreachable!()
     }
 
-    fn git_diff_recalc(
-        &mut self,
-        project: ModelHandle<Project>,
-        cx: &mut ViewContext<Self>,
-    ) -> Task<Result<()>> {
-        self.editor
-            .update(cx, |editor, cx| editor.git_diff_recalc(project, cx))
-    }
-
     fn to_item_events(event: &Self::Event) -> SmallVec<[ItemEvent; 2]> {
         Editor::to_item_events(event)
     }