Drop support legacy waiter system (#477)

addon/addon/waiter-manager.ts

@@ -1,8 +1,5 @@
 import { PendingWaiterState, Waiter, WaiterName } from './types';
 
-import Ember from 'ember';
-import { registerWaiter } from '@ember/test';
-
 type Indexable = Record<any, unknown>;
 
 // this ensures that if @ember/test-waiters exists in multiple places in the
@@ -37,18 +34,6 @@ function getGlobal(): Indexable {
   throw new Error('unable to locate global object');
 }
 
-/**
- * Backwards compatibility with legacy waiters system.
- *
- * We want to always register a waiter using the legacy waiter system, as right
- * now if consumers are not on the right version of @ember/test-helpers, using
- * this addon will result in none of these waiters waiting.
- */
-// eslint-disable-next-line ember/new-module-imports
-if (Ember.Test) {
-  registerWaiter(() => !hasPendingWaiters());
-}
-
 /**
  * Registers a waiter.
  *