CRA: unregister ServiceWorker

#### Step1:

In app entry, comment `registerServiceWorker()`

#### Step2:

Your service worker file is supposed to be in: `https://yourdomain.com/service-worker.js` and this file is never 
chacehd at yoyr users computer for offline use. 

so after commenting the `registerServiceWorker` in your CRA app entry, the 
CRA shoud not generate `service-worker.js` for you. What you should do is make one yourself and write inside 
it the code you will use to unregsister your clients:

```
// unregister SW
 navigator.serviceWorker.getRegistrations()
  .then((registrations) => {
    const unregisterPromise = registrations.map((registration) => {
    return registration.unregister();
  })
});

// cleare chaces
window.caches.keys()
  .then((cacheNames) => {
    return Promise.all(cacheNames.map((cacheName) => {
    return window.caches.delete(cacheName);
  }));
});
```