[RN] Remove no longer needed fetch API fallback

react/features/base/util/loadScript.native.js

 /**
  * Loads a script from a specific URL. React Native cannot load a JS
- * file/resource/URL via a <script> HTML element, so the implementation
- * fetches the specified src as plain text (e.g. via XMLHttpRequest) and then
+ * file/resource/URL via a <script> HTML element, so the implementation
+ * fetches the specified src as plain text using fetch() and then
  * evaluates the fetched string as JavaScript code (i.e. via the {@link eval}
  * function).
  *
  * @returns {void}
  */
 export function loadScript(url) {
-    let fetch;
-    const method = 'GET';
-
-    // Prefer the Fetch API. Apart from the fact that we're fetching the
-    // specified script as a static resource, the Fetch API provides more
-    // detailed errors.
-    if (typeof (fetch = window.fetch) === 'function') {
-        fetch = fetch(url, { method });
-    } else {
-        // Otherwise, fall back to the XMLHttpRequest API.
-        fetch
-            = new Promise(resolve => {
-                const xhr = new XMLHttpRequest();
-
-                xhr.responseType = 'text';
-
-                xhr.onreadystatechange = () => {
-                    if (xhr.readyState === 4) {
-                        resolve(xhr);
-                    }
-                };
-
-                xhr.open(method, url, /* async */ true);
-                xhr.send();
-            });
-    }
-
     return (
-        fetch
+        fetch(url, { method: 'GET' })
             .then(response => {
                 switch (response.status) {
                 case 200: