Simplify CronTrigger methods list & its usage

[MAILPOET-2538]

lib/Cron/CronTrigger.php

@@ -12,12 +12,13 @@ class CronTrigger {
   const METHOD_MAILPOET = 'MailPoet';
   const METHOD_WORDPRESS = 'WordPress';
 
-  public static $available_methods = [
+  const METHODS = [
     'mailpoet' => self::METHOD_MAILPOET,
     'wordpress' => self::METHOD_WORDPRESS,
     'linux_cron' => self::METHOD_LINUX_CRON,
     'none' => 'Disabled',
   ];
+
   const DEFAULT_METHOD = 'WordPress';
   const SETTING_NAME = 'cron_trigger';
 
@@ -36,8 +37,4 @@ class CronTrigger {
       // cron exceptions should not prevent the rest of the site from loading
     }
   }
-
-  static function getAvailableMethods() {
-    return self::$available_methods;
-  }
 }