[assert helpers] Remove toWarnDev from fixtures/dom (facebook#31894)

This is unused and never was:
facebook@e6a0473

Author: rickhanlonii

fixtures/dom/src/__tests__/nested-act-test.js

@@ -14,8 +14,6 @@ let TestAct;
 
 global.__DEV__ = process.env.NODE_ENV !== 'production';
 
-expect.extend(require('../toWarnDev'));
-
 describe('unmocked scheduler', () => {
   beforeEach(() => {
     jest.resetModules();