Remove credit card samples (#896)

* removed credit card payment samples
* update docs to reflect sample changes
* update readme to recommend braintree for direct card payments

sample/payments/AuthorizationCapture.php

@@ -5,18 +5,22 @@
 // API used: /v1/payments/payment
 // https://developer.paypal.com/webapps/developer/docs/api/#capture-an-authorization
 
-/** @var Authorization $authorization */
-$authorization = require 'GetAuthorization.php';
 use PayPal\Api\Amount;
 use PayPal\Api\Authorization;
 use PayPal\Api\Capture;
 
+
+// Replace $authorizationId with any static Id you might already have. 
+$authorizationId = "<your authorization id here>";
+
 // ### Capture Payment
 // You can capture and process a previously created authorization
 // by invoking the $authorization->capture method
 // with a valid ApiContext (See bootstrap.php for more on `ApiContext`)
 try {
-    $authId = $authorization->getId();
+
+    // Retrieve the authorization
+    $authorization = Authorization::get($authorizationId, $apiContext);
 
     $amt = new Amount();
     $amt->setCurrency("USD")