Technical debt: `Service#disabled_title` no longer used and is left dangling in `services/_form.html.haml`

What

I propose we remove the unused variables from the view and then check individual services to see if they use disabled_title elsewhere

Why

The following discussion from gitlab-ee!4591 should be addressed:

• @jamedjo started a discussion: (+2 comments)

  @kushalpandya Do we still use disabled_title in services?

  It looks like you removed this from app/views/projects/services/_form.html.haml in gitlab-ee@fc2cc1e235f9d760b9b69fe5911f8e4396ab8965, but we still set a variable in that view which doesn't appear to ever be used.

  We also set this method on lots of services still. I'll probably leave this here, as I've added a raise disabled_title unless pipeline elsewhere, but we might be able to remove this method from most other services if not used.

Follow up from @jarka identified the MR where this changed:

@kushalpandya Just a quick check - we changed testing/saving services in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11599. I can see there we removed disabled_title from one view (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11599/diffs#469e0c28b0fa25f5b73ad1405d523f07ffd90871_20_25 - btw. I see we still assign the variable in that diff - maybe it could be re-checked and removed?) but that's all. I don't remember any other changes of disabled_title (I didn't even remember this one, just found it in the diff)