Remove iOS instructions from the maps plugin readme. (#744)

This was a bit confusing, removing until the plugin supports iOS.

Author: amirh

packages/google_maps_flutter/README.md

@@ -30,8 +30,6 @@ as a [dependency in your pubspec.yaml file](https://flutter.io/platform-plugins/
 
 Get an API key at <https://cloud.google.com/maps-platform/>.
 
-### Android
-
 Specify your API key in the application manifest `android/app/src/main/AndroidManifest.xml`:
 
 ```xml
@@ -41,28 +39,6 @@ Specify your API key in the application manifest `android/app/src/main/AndroidMa
                android:value="YOUR KEY HERE"/>
 ```
 
-### iOS
-
-Supply your API key in the application delegate `ios/Runner/AppDelegate.m`:
-
-```objectivec
-#include "AppDelegate.h"
-#include "GeneratedPluginRegistrant.h"
-#import "GoogleMaps/GoogleMaps.h"
-
-@implementation AppDelegate
-
-- (BOOL)application:(UIApplication *)application
-    didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
-  [GMSServices provideAPIKey:@"YOUR KEY HERE"];
-  [GeneratedPluginRegistrant registerWithRegistry:self];
-  return [super application:application didFinishLaunchingWithOptions:launchOptions];
-}
-@end
-```
-
-### Both
-
 You can now add a `GoogleMap` widget to your widget tree.
 
 The map view can be controlled with the `GoogleMapController` that is passed to