Inline generateToken for clarity

The only thing Security::generateToken was providing was a default value
 for the $action, which created a pattern of using the same $action
 everywhere, which may not be the best way to go.

 Since it was essentially a wrapper for WP's built-in nonce functions,
 it seemed clearer to use those functions directly to be more explicit
 about how we're handling tokens.

[MAILPOET-2030]

mailpoet/tests/integration/API/JSON/APITest.php

@@ -73,11 +73,15 @@ class APITest extends \MailPoetTest {
         expect($api instanceof JSONAPI)->true();
       }
     );
+    $wpStub = Stub::make(new WPFunctions, [
+      'wpVerifyNonce' => asCallable(function() {
+        return true;
+      })]);
     $api = Stub::makeEmptyExcept(
       $this->api,
       'setupAjax',
       [
-        'wp' => new WPFunctions,
+        'wp' => $wpStub,
         'processRoute' => Stub::makeEmpty(new SuccessResponse),
         'settings' => $this->container->get(SettingsController::class),
       ]