Fix RemoveAllCommand return type

The command should always return an integer as exit code.
The called method does not return anything, resulting in null to be
returned.

Classes/Command/RemoveAllCommand.php

@@ -22,8 +22,10 @@ class RemoveAllCommand extends Command
     {
         Bootstrap::initializeBackendAuthentication();
 
-        return GeneralUtility::makeInstance(ObjectManager::class)
+        GeneralUtility::makeInstance(ObjectManager::class)
             ->get(CleanupService::class)
             ->deleteAllData();
+
+        return 0;
     }
 }