docs(app, ios): remove nil check prior to configure

It appears that checking `[FIRApp defaultApp] == nil` will log an error message,
and I'm not sure how it could ever be called twice (which would throw an exception instead)?
Conforms with upstream documentation https://firebase.google.com/docs/ios/setup#objective-c
Fixes #5739

Author: mikehardy

docs/index.md

@@ -112,9 +112,7 @@ Within your existing `didFinishLaunchingWithOptions` method, add the following t
 ```
 - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
   // Add me --- \/
-  if ([FIRApp defaultApp] == nil) {
-    [FIRApp configure];
-  }
+  [FIRApp configure];
   // Add me --- /\
   // ...
 }