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

tests/test-archive.sh

@@ -1,29 +0,0 @@
-#!/bin/bash
-#
-# Copyright (C) 2011 Colin Walters <walters@verbum.org>
-#
-# This library is free software; you can redistribute it and/or
-# modify it under the terms of the GNU Lesser General Public
-# License as published by the Free Software Foundation; either
-# version 2 of the License, or (at your option) any later version.
-#
-# This library is distributed in the hope that it will be useful,
-# but WITHOUT ANY WARRANTY; without even the implied warranty of
-# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
-# Lesser General Public License for more details.
-#
-# You should have received a copy of the GNU Lesser General Public
-# License along with this library; if not, write to the
-# Free Software Foundation, Inc., 59 Temple Place - Suite 330,
-# Boston, MA 02111-1307, USA.
-
-set -e
-
-. $(dirname $0)/libtest.sh
-
-echo '1..11'
-
-setup_test_repository "archive"
-echo "ok setup"
-
-. ${SRCDIR}/archive-test.sh