[RN] Remove no longer needed polyfills (#3377)

- navigator.{platform,plugins} were needed by the no longer existing screenshare
  adapter
- document.implementation is already polyfilled by xmldom

react/features/base/lib-jitsi-meet/native/polyfills-browser.js

     //
     // Required by:
     // - jQuery
-    // - lib-jitsi-meet/modules/RTC/adapter.screenshare.js
     // - Strophe
     if (typeof global.document === 'undefined') {
         const document
             document.cookie = '';
         }
 
-        // document.implementation
-        //
-        // Required by:
-        // - jQuery
-        if (typeof document.implementation === 'undefined') {
-            document.implementation = {};
-        }
-
         // document.implementation.createHTMLDocument
         //
         // Required by:
     const { navigator } = global;
 
     if (navigator) {
-        // platform
-        //
-        // Required by:
-        // - lib-jitsi-meet/modules/RTC/adapter.screenshare.js
-        if (typeof navigator.platform === 'undefined') {
-            navigator.platform = '';
-        }
-
-        // plugins
-        //
-        // Required by:
-        // - lib-jitsi-meet/modules/RTC/adapter.screenshare.js
-        if (typeof navigator.plugins === 'undefined') {
-            navigator.plugins = [];
-        }
-
         // userAgent
         //
         // Required by:
-        // - lib-jitsi-meet/modules/RTC/adapter.screenshare.js
         // - lib-jitsi-meet/modules/browser/BrowserDetection.js
         let userAgent = navigator.userAgent || '';