Following these two comments:
* https://github.com/hypothesis/client/pull/3929/files#r750399422
* https://github.com/hypothesis/client/pull/3929/files#r750414036

I am proposing in this PR to remove the anti-collision field `authority`
from the message that is send to discover frames. I believe the message
contains enough information to avoid to be confused by other unrelated
`postMessage` in the host frame.