docs(readme): remove documentation for old visrec 'apikey'

Author: dpopp07

README.md

@@ -84,7 +84,6 @@ Watson services are migrating to token-based Identity and Access Management (IAM
 
 - With some service instances, you authenticate to the API by using **[IAM](#iam)**.
 - In other instances, you authenticate by providing the **[username and password](#username-and-password)** for the service instance.
-- Visual Recognition uses a form of [API key](#api-key) only with instances created before May 23, 2018. Newer instances of Visual Recognition use [IAM](#iam).
 
 ### Getting credentials
 To find out which authentication to use, view the service credentials. You find the service credentials for authentication the same way for all Watson services:
@@ -148,19 +147,6 @@ var discovery = new DiscoveryV1({
   });
 ```
 
-### API key
-
-**Important**: This type of authentication works only with Visual Recognition instances created before May 23, 2018. Newer instances of Visual Recognition use [IAM](#iam).
-
-```javascript
-var VisualRecognitionV3 = require('watson-developer-cloud/visual-recognition/v3');
-
-var visualRecognition = new VisualRecognitionV3({
-    version: '{version}',
-    api_key: '{api_key}'
-  });
-```
-
 ### Sending request headers
 
 Custom headers can be passed with any request. Each method has an optional parameter `headers` which can be used to pass in these custom headers, which can override headers that we use as parameters.