Change: remove prototype instanceof Object check*

* It didn't work as expected in Waterfox Classic v2019.10, which is based on Firefox v56:
```
prototype.__proto__: HTMLElementPrototype
prototype instanceof Object: false
prototype instanceof HTMLElement: true
prototype instanceof Element: false
HTMLElement instanceof Object: true
Element instanceof Object: false
```

Chrome v49, v79:
```
prototype.__proto__: Element
prototype instanceof Object: true
prototype instanceof HTMLElement: true
prototype instanceof Element: true
HTMLElement instanceof Object: true
Element instanceof Object: true
```

See PoziWorld/Scroll-To-Top-Button-extension#7

packages/custom-elements/src/CustomElementRegistry.js

@@ -168,9 +168,6 @@ export default class CustomElementRegistry {
     try {
       /** @type {!Object} */
       const prototype = constructor.prototype;
-      if (!(prototype instanceof Object)) {
-        throw new TypeError('The custom element constructor\'s prototype is not an object.');
-      }
 
       function getCallback(name) {
         const callbackValue = prototype[name];