revert the process to original code

Author: go-to-k

packages/aws-cdk/lib/cxapp/cloud-assembly.ts

@@ -119,9 +119,6 @@ export class CloudAssembly extends BaseStackAssembly {
   ): Promise<StackCollection> {
     if (topLevelStacks.length > 0) {
       return this.extendStacks(topLevelStacks, stacks, extend);
-    } else if (stacks.length > 0) {
-      // Fallback to all stacks if no top-level stacks (e.g., app including stages only in top-level assembly)
-      return new StackCollection(this, stacks);
     } else {
       throw new ToolkitError('No stack found in the main cloud assembly. Use "list" to print manifest');
     }