Remove errornous "folder exists" check in backup health check

This code was buggy for some storages, and useless anyway: If the
storage base directory did not exist, retrieving the backup list would
have failed altogether, and the check returned error.

aleksis/core/health_checks.py

@@ -34,9 +34,6 @@ class BaseBackupHealthCheck(BaseHealthCheckBackend):
     def check_status(self):
         storage = get_storage()
         backups = storage.list_backups(content_type=self.content_type)
-        if not storage.storage.exists(""):
-            self.add_error(_("The backup folder doesn't exist."))
-            return
         if backups:
             last_backup = backups[:1]
             last_backup_time = dbbackup_utils.filename_to_date(last_backup[0])