chore: remove duplicated FocusKeyManager

Removes the FocusKeyManager class that was duplicated from the CDK.

Author: crisbeto

src/lib/core/a11y/focus-key-manager.ts

@@ -6,27 +6,4 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {ListKeyManager, ListKeyManagerOption} from './list-key-manager';
-
-/**
- * This is the interface for focusable items (used by the FocusKeyManager).
- * Each item must know how to focus itself, whether or not it is currently disabled
- * and be able to supply it's label.
- */
-export interface FocusableOption extends ListKeyManagerOption {
-  focus(): void;
-}
-
-export class FocusKeyManager<T> extends ListKeyManager<FocusableOption & T> {
-  /**
-   * This method sets the active item to the item at the specified index.
-   * It also adds focuses the newly active item.
-   */
-  setActiveItem(index: number): void {
-    super.setActiveItem(index);
-
-    if (this.activeItem) {
-      this.activeItem.focus();
-    }
-  }
-}
+export {FocusKeyManager, FocusableOption} from '@angular/cdk/a11y';