Remove vendored fetch polyfill, update to whatwg-fetch@3.0

Author: janicduplessis

Libraries/Core/setUpGlobals.js

@@ -21,6 +21,10 @@ if (global.window === undefined) {
   global.window = global;
 }
 
+if (global.self === undefined) {
+  global.self = global;
+}
+
 // Set up process
 global.process = global.process || {};
 global.process.env = global.process.env || {};