We use singular form for delete command argument in all places.
This commit replaces conntrack-helper-ids with a singular form.
The only visible change is a fix for the help message below.
openstack network l3 conntrack helper delete
<router>
<conntrack-helper-ids>
[<conntrack-helper-ids> ...]
Change-Id: I50bbd9f6199071bb86cbb2f37c45ebda1de58433