feat: removed option to add a start or end node to the editor

Start and End nodes indicate the start and endpoint for program evaluation, for now adding more start points and end points would overcomplicate the initial implementation. Supporting multiple end nodes could be an interesting option to look into later in development.

ref: N25B-114

src/visualProgrammingUI/components/DragDropSidebar.tsx

@@ -124,12 +124,6 @@ export function Sidebar() {
             <div className="description">
                 You can drag these nodes to the pane to create new nodes.
             </div>
-            <DraggableNode className="start" nodeType="start" onDrop={handleNodeDrop}>
-                start Node
-            </DraggableNode>
-            <DraggableNode className="end" nodeType="end" onDrop={handleNodeDrop}>
-                end Node
-            </DraggableNode>
             <DraggableNode className="phase" nodeType="phase" onDrop={handleNodeDrop}>
                 phase Node
             </DraggableNode>