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/lib/Util/Security.php

@@ -7,7 +7,6 @@ use MailPoet\Entities\NewsletterEntity;
 use MailPoet\Entities\SubscriberEntity;
 use MailPoet\Newsletter\NewslettersRepository;
 use MailPoet\Subscribers\SubscribersRepository;
-use MailPoet\WP\Functions as WPFunctions;
 
 class Security {
   const HASH_LENGTH = 12;
@@ -27,10 +26,6 @@ class Security {
     $this->subscribersRepository = $subscribersRepository;
   }
 
-  public static function generateToken($action = 'mailpoet_token') {
-    return WPFunctions::get()->wpCreateNonce($action);
-  }
-
   /**
    * Generate random lowercase alphanumeric string.
    * 1 lowercase alphanumeric character = 6 bits (because log2(36) = 5.17)