Remove users' group memberships when they're deleted. And don't remove groups they're the owner of.

'Owner' is a hangover from the old group stuff anyway. It doesn't make sense to delete a group just because a group admin has been deleted.

htdocs/lib/user.php

@@ -669,8 +669,10 @@ function delete_user($userid) {
     SET email = email || ?
     WHERE owner = ?', array($emailsuffix, $userid));
 
-    // mark all groups the user owns as deleted
-    set_field('group', 'deleted', '1', 'owner', $userid);
+    // Remove user from any groups they're in, invited to or want to be in
+    delete_records('group_member', 'member', $userid);
+    delete_records('group_member_request', 'member', $userid);
+    delete_records('group_member_invite', 'member', $userid);
 
     db_commit();