chore: remove references to the web animations polyfill

Gets rid of the Web animations polyfill from our demo apps, as well as from the docs. The polyfill is not longer required as of Angular 6.

Author: crisbeto

guides/getting-started.md

@@ -48,10 +48,6 @@ npm install --save @angular/animations
 yarn add @angular/animations
 ```
 
-**Note:** `@angular/animations` uses the WebAnimation API that isn't supported by all browsers yet.
-If you want to support Material component animations in these browsers, you'll have to
-[include a polyfill](https://github.com/web-animations/web-animations-js).
-
 ```ts
 import {BrowserAnimationsModule} from '@angular/platform-browser/animations';