We used Clear-Site-Data to cushion transition period for local gateway
exposed at http://localhost while we were still figuring out
security-related details.
In the final implementation subdomain gateways are not tied to a
hostname explicitly, which removes the risk of cookies leaking,
removing the need for the header.
Turns out it causes issues for Firefox users, so let's just remove it.
Closes https://github.com/ipfs-shipyard/ipfs-companion/issues/977
d61ae2bcb4