chore: fix lint warnings (#9869)

Fixes a few lint warnings that got into master.

src/cdk/a11y/live-announcer/live-announcer.spec.ts

@@ -73,7 +73,7 @@ describe('LiveAnnouncer', () => {
 
     it('should return a promise that resolves after the text has been announced', fakeAsync(() => {
       const spy = jasmine.createSpy('announce spy');
-      const promise = announcer.announce('something').then(spy);
+      announcer.announce('something').then(spy);
 
       expect(spy).not.toHaveBeenCalled();
       tick(100);

src/lib/tabs/tab-body.ts

@@ -24,14 +24,7 @@ import {
   forwardRef,
   ViewChild,
 } from '@angular/core';
-import {
-  trigger,
-  state,
-  style,
-  animate,
-  transition,
-  AnimationEvent,
-} from '@angular/animations';
+import {AnimationEvent} from '@angular/animations';
 import {TemplatePortal, CdkPortalOutlet, PortalHostDirective} from '@angular/cdk/portal';
 import {Directionality, Direction} from '@angular/cdk/bidi';
 import {Subscription} from 'rxjs/Subscription';

src/lib/tabs/tab.ts

@@ -51,7 +51,7 @@ export class MatTab extends _MatTabMixinBase implements OnInit, CanDisable, OnCh
    */
   @ContentChild(MatTabContent, {read: TemplateRef}) _explicitContent: TemplateRef<any>;
 
-  /** Template inside the MatTab view that contains an <ng-content>. */
+  /** Template inside the MatTab view that contains an `<ng-content>`. */
   @ViewChild(TemplateRef) _implicitContent: TemplateRef<any>;
 
   /** The plain text label for the tab, used when there is no template label. */