Remove the "cost involved" comment from the onboarding page

The "Onboarding new starters" page ends with:

If working on Gitlab deployment and/or MWSv3:

* Add to [GitHub organisation](https://github.com/uisautomation) (_cost involved_)

The "cost involved" comment should be removed as it is no longer accurate. This also means the item could be moved into the main checklist as cost is not a gating factor.