re-add lost group_delete js event

There used to be a js event called group_delete, a while ago this went missing while the events group_insert and group_update remain.

I found the old event here https://github.com/roundcube/roundcubemail/blob/c8a714cca4978395292e82ed1971fbf92fa78487/program/js/app.js#L4496 but cannot find exactly which commit removed it.