workspace: Add Close Multibuffers pane context menu entry (#39199)

Release Notes:

- N/A *or* Added/Fixed/Improved ...

crates/git_ui/src/commit_view.rs

@@ -452,10 +452,6 @@ impl Item for CommitView {
             .update(cx, |editor, cx| editor.deactivated(window, cx));
     }
 
-    fn is_singleton(&self, _: &App) -> bool {
-        false
-    }
-
     fn act_as_type<'a>(
         &'a self,
         type_id: TypeId,

crates/git_ui/src/file_diff_view.rs

@@ -263,10 +263,6 @@ impl Item for FileDiffView {
             .update(cx, |editor, cx| editor.deactivated(window, cx));
     }
 
-    fn is_singleton(&self, _: &App) -> bool {
-        false
-    }
-
     fn act_as_type<'a>(
         &'a self,
         type_id: TypeId,

crates/git_ui/src/project_diff.rs

@@ -613,10 +613,6 @@ impl Item for ProjectDiff {
         self.editor.for_each_project_item(cx, f)
     }
 
-    fn is_singleton(&self, _: &App) -> bool {
-        false
-    }
-
     fn set_nav_history(
         &mut self,
         nav_history: ItemNavHistory,

crates/git_ui/src/text_diff_view.rs

@@ -324,10 +324,6 @@ impl Item for TextDiffView {
             .update(cx, |editor, cx| editor.deactivated(window, cx));
     }
 
-    fn is_singleton(&self, _: &App) -> bool {
-        false
-    }
-
     fn act_as_type<'a>(
         &'a self,
         type_id: TypeId,