fetcher: Drop the libsoup queue

fetcher: Drop the libsoup queue

Now that we have queuing in the higher level pull logic, we don't
need to do this anymore.

It's tempting to keep it since the code diff is so small (without
completely rewriting things), but dropping it here will make
it easier to see when things go wrong at a higher level.

Note that I kept an assertion.

Closes: #654
Approved by: jlebon