refactor: removed start and end nodes from the addNode functionality as they are not supported for manual adding

removed unnecessary code.

BREAKING: end and start nodes are no longer supported by the dnd addNode functionality.

ref: N25B-114

src/pages/VisProgPage/visualProgrammingUI/components/DragDropSidebar.tsx

@@ -80,23 +80,9 @@ function addNode(nodeType: string, position: XYPosition) {
           id: `phase-${phaseNumber}`,
           type: nodeType,
           position,
-          data: {label: `"new"`, number: phaseNumber},
+          data: {label: 'new', number: phaseNumber},
         };
       }
-      case "start":
-        return {
-          id: getId(),
-          type: nodeType,
-          position,
-          data: {label: `new start node`},
-        };
-      case "end":
-        return {
-          id: getId(),
-          type: nodeType,
-          position,
-          data: {label: `new end node`},
-        };
       case "norm":
       {
         const normNumber = nds.filter((node) => node.type === 'norm').length;