fix(cdk/tree): fix unused error

Author: BobobUnicorn

src/components-examples/cdk/tree/cdk-tree-redesign/cdk-tree-flat-level-accessor-example.ts

@@ -1,6 +1,6 @@
 import {ArrayDataSource} from '@angular/cdk/collections';
 import {Component} from '@angular/core';
-import {FlatFoodNode, FLAT_DATA, NESTED_DATA} from './tree-data';
+import {FlatFoodNode, FLAT_DATA} from './tree-data';
 
 /**
  * @title Tree with flat nodes