Remove re-engagement-email feature flag

[MAILPOET-3859]

lib/Features/FeaturesController.php

@@ -5,13 +5,10 @@ namespace MailPoet\Features;
 use MailPoetVendor\Doctrine\DBAL\Exception\TableNotFoundException;
 
 class FeaturesController {
-  const RE_ENGAGEMENT_EMAIL = 're-engagement-email';
 
   // Define feature defaults in the array below in the following form:
   //   self::FEATURE_NAME_OF_FEATURE => true,
-  private $defaults = [
-    self::RE_ENGAGEMENT_EMAIL => false,
-  ];
+  private $defaults = [];
 
   /** @var array */
   private $flags;