Issue #2106709 by tim.plunkett: Remove legacy router backwards compatibility layer.

core/core.services.yml

core/includes/ajax.inc

core/includes/menu.inc

.../lib/Drupal/Core/Authentication/AuthenticationManager.php

...pendencyInjection/Compiler/RegisterRouteEnhancersPass.php

...ib/Drupal/Core/EventSubscriber/LegacyAccessSubscriber.php

...rupal/Core/EventSubscriber/LegacyControllerSubscriber.php

core/lib/Drupal/Core/LegacyUrlMatcher.php

...odules/menu_link/lib/Drupal/menu_link/Entity/MenuLink.php

...rupal/menu_link/Tests/Plugin/Core/Entity/MenuLinkTest.php

...s/system/lib/Drupal/system/PathBasedBreadcrumbBuilder.php

core/modules/system/system.api.php