apiFetch: Use URL computation in apiFetch.

For #4146, but also to fix a recent regression from 865914f.

The URL constructor already throws a special "Invalid URL" error if
it can't put together a valid URL from the inputs, so, nicely, we
get to remove the `isValidUrl` call site.

`isValidUrl` failed to alert us of a double-forward-slash issue in
this string concatenation: the realm gained a trailing slash in its
new spelling as `realm.origin` from 865914f, and we kept the
leading slash in what we were adding to the end of the realm. This
is a sign that `isValidUrl` should be retired ASAP; we'll do that in
an upcoming commit.

src/api/apiFetch.js

@@ -3,7 +3,7 @@ import * as Sentry from '@sentry/react-native';
 import type { UrlParams } from '../utils/url';
 import type { Auth } from './transportTypes';
 import { getAuthHeaders } from './transport';
-import { encodeParamsForUrl, isValidUrl } from '../utils/url';
+import { encodeParamsForUrl } from '../utils/url';
 import userAgent from '../utils/userAgent';
 import { networkActivityStart, networkActivityStop } from '../utils/networkActivity';
 import { makeErrorFromApi } from './apiErrors';
@@ -34,15 +34,7 @@ export const apiFetch = async (
   auth: Auth,
   route: string,
   params: $Diff<$Exact<RequestOptions>, {| headers: mixed |}>,
-) => {
-  const url = `${auth.realm.toString()}/${apiVersion}/${route}`;
-
-  if (!isValidUrl(url)) {
-    throw new Error(`Invalid url ${url}`);
-  }
-
-  return fetch(url, getFetchParams(auth, params));
-};
+) => fetch(new URL(`/${apiVersion}/${route}`, auth.realm).toString(), getFetchParams(auth, params));
 
 export const apiCall = async (
   auth: Auth,