docs(material/tree): delete tree-checklist example. (#27577)

Delete tree-checklist example. That's because this example doesn't
correctly follow [ARIA Tree View
interaction](https://www.w3.org/WAI/ARIA/apg/patterns/treeview/) for
selection. A correct solution would use aria-selected or aria-checked.

(cherry picked from commit 7bb4491)

src/components-examples/material/tree/index.ts

@@ -1,4 +1,3 @@
-export {TreeChecklistExample} from './tree-checklist/tree-checklist-example';
 export {TreeDynamicExample} from './tree-dynamic/tree-dynamic-example';
 export {TreeFlatOverviewExample} from './tree-flat-overview/tree-flat-overview-example';
 export {TreeHarnessExample} from './tree-harness/tree-harness-example';

src/dev-app/tree/tree-demo.html

@@ -15,10 +15,6 @@
     <mat-expansion-panel-header>CDK Nested tree</mat-expansion-panel-header>
     <cdk-tree-nested-example></cdk-tree-nested-example>
   </mat-expansion-panel>
-  <mat-expansion-panel>
-    <mat-expansion-panel-header>Todo list tree</mat-expansion-panel-header>
-    <tree-checklist-example></tree-checklist-example>
-  </mat-expansion-panel>
   <mat-expansion-panel>
     <mat-expansion-panel-header>Dynamic flat tree</mat-expansion-panel-header>
     <tree-dynamic-example></tree-dynamic-example>

src/dev-app/tree/tree-demo.ts

@@ -10,7 +10,6 @@ import {CdkTreeModule} from '@angular/cdk/tree';
 import {CommonModule} from '@angular/common';
 import {CdkTreeFlatExample, CdkTreeNestedExample} from '@angular/components-examples/cdk/tree';
 import {
-  TreeChecklistExample,
   TreeDynamicExample,
   TreeFlatOverviewExample,
   TreeHarnessExample,
@@ -38,7 +37,6 @@ import {MatTreeModule} from '@angular/material/tree';
     CdkTreeNestedExample,
     CommonModule,
     FormsModule,
-    TreeChecklistExample,
     TreeDynamicExample,
     TreeFlatOverviewExample,
     TreeHarnessExample,