fix(cdk/tree): remove FocusMonitor from CdkTreeModule providers

Author: annieyw

src/cdk/tree/tree-module.ts

@@ -6,7 +6,6 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {FocusMonitor} from '@angular/cdk/a11y';
 import {NgModule} from '@angular/core';
 import {CdkTreeNodeOutlet} from './outlet';
 import {CdkTreeNodePadding} from './padding';
@@ -28,6 +27,5 @@ const EXPORTED_DECLARATIONS = [
 @NgModule({
   exports: EXPORTED_DECLARATIONS,
   declarations: EXPORTED_DECLARATIONS,
-  providers: [FocusMonitor]
 })
 export class CdkTreeModule {}