Remove String.prototype.contains polyfill

Summary:
Now that String.prototype.includes is there, we should remove the .contains one which has not been standardized.

For fb reviewers, this needs to land after D2910339 which updates internal callsites.
Closes facebook/react-native#5794

Reviewed By: svcscm

Differential Revision: D2910855

Pulled By: vjeux

fb-gh-sync-id: 8fd216222385f038995d1ed10e8a2c4c34c7e928

react-packager/src/Resolver/polyfills/String.prototype.es6.js

@@ -44,19 +44,6 @@ if (!String.prototype.endsWith) {
   };
 }
 
-if (!String.prototype.contains) {
-  String.prototype.contains = function(search) {
-    'use strict';
-    if (this == null) {
-      throw TypeError();
-    }
-    var string = String(this);
-    var pos = arguments.length > 1 ?
-      (Number(arguments[1]) || 0) : 0;
-    return string.indexOf(String(search), pos) !== -1;
-  };
-}
-
 if (!String.prototype.repeat) {
   String.prototype.repeat = function(count) {
     'use strict';