Clean up webview readme (flutter#3900)

Author: Emmanuel Garcia

packages/webview_flutter/README.md

@@ -68,17 +68,6 @@ class WebViewExampleState extends State<WebViewExample> {
 }
 ```
 
-`SurfaceAndroidWebView()` requires [API level 19](https://developer.android.com/studio/releases/platforms?hl=th#4.4). The plugin itself doesn't enforce the API level, so if you want to make the app available on devices running this API level or above, add the following to `<your-app>/android/app/build.gradle`:
-
-```gradle
-android {
-    defaultConfig {
-        // Required by the Flutter WebView plugin.
-        minSdkVersion 19
-    }
-}
-```
-
 #### Enable Material Components for Android
 
 To use Material Components when the user interacts with input elements in the WebView,