lib/pull: Drop partial fetch code from libsoup backend

lib/pull: Drop partial fetch code from libsoup backend

Doing this in prep for libglnx tmpdir porting, but I think we should also do
this because the partial fetch code IMO was never fully baked; among other
things it was never integrated into the scheme we came up with for "boot id
sync" that we use for complete/staged objects.

There's a lot of complexity here that while we have some coverage for, I think
we need to refocus on the core functionality. The libcurl backend doesn't have
an equivalent to this today.

In particular for small objects, this is simply overly complex. The downside is
clearly for large objects like FAH's 61MB initramfs; not being able to resume
fetches of those is unfortunate.

In practice though, I think most people should be using deltas, and we need to
make sure deltas work for large objects anyways.

Further ultimately the peer-to-peer work should help a lot for people
with truly unreliable connections.

Closes: #1176
Approved by: jlebon