Removed redundant string cast, now correctly identified by vimeo/psalm:^4.20

`substr()` only fails when one of the inputs is not a `string`, but that no longer
holds true as per `php:^8.0`, since the input must be a `string` upfront.

src/Provider/Dce/SystemDceSecurityProvider.php

@@ -229,6 +229,6 @@ class SystemDceSecurityProvider implements DceSecurityProviderInterface
             return '';
         }
 
-        return trim((string) substr($sid, $lastHyphen + 1));
+        return trim(substr($sid, $lastHyphen + 1));
     }
 }