core: Remove old "archive" mode

We'll always have "bare" mode for keeping files-as-hardlinks as root.
But "archive" was my second attempt at a format for non-root file
storage, used by the gnome-ostree buildsystem which runs as non-root.

It was really handy to have a "tar" like mode where I can create
tarballs as a user, that contain files owned by root for example.

The "archive" mode stored content files as two pieces in the
filesystem; ".file" contained metadata, and ".filecontent" was the
actual content, uncompressed.  The nice thing about this was that to
check out a tree as non-root, you could just hardlink into the repo.

However, archive was fairly bad for serving via HTTP; it required
*two* HTTP requests per content object, greatly magnifing the already
inefficient fetch process.  So "archive-z2" was introduced.

To allow gnome-ostree to still check out trees as a user, the
"uncompressed-object-cache" was introduced, and that's how things have
been working for a while.

So we should just be able to kill this code.  Specifically note just
how much better the stage_object() function became.

https://bugzilla.gnome.org/show_bug.cgi?id=706057