chore: remove duplicate imports (#1701)

Author: crisbeto

src/lib/core/a11y/index.ts

@@ -3,10 +3,6 @@ import {FocusTrap} from './focus-trap';
 import {MdLiveAnnouncer} from './live-announcer';
 import {InteractivityChecker} from './interactivity-checker';
 
-export {FocusTrap} from './focus-trap';
-export {MdLiveAnnouncer} from './live-announcer';
-export {InteractivityChecker} from './interactivity-checker';
-
 export const A11Y_PROVIDERS = [MdLiveAnnouncer, InteractivityChecker];
 
 @NgModule({