Need to implement hook_uninstall to delete the variable

Hi,

When we check and configure "Automatically backup permission every-time permissions are updated" from (/people/permissions/backup_permissions) and save a configuration, this saved the variable "backup_permissions_auto_backup_config" and the same is not deleted when we uninstall the module.

Proposed Solution:
1. In the backup_permissions.install, implement hook_uninstall().