Drop bullseye-updates APT source

It prevents me from building 6.0:

  E: The repository 'http://tagged.snapshots.deb.tails.boum.org/6.0/debian bullseye-updates Release' does not have a Release file.

We added it for our OnionShare forward-port (#20135, !1375) just in
case any of the dependencies from Bullseye that we install gets an
update. We are tracking this manually any way, so we should be fine.