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

src/libostree/ostree-repo-checkout.c

@@ -188,9 +188,6 @@ find_loose_for_checkout (OstreeRepo             *self,
         case OSTREE_REPO_MODE_BARE:
           path = _ostree_repo_get_object_path (self, checksum, OSTREE_OBJECT_TYPE_FILE);
           break;
-        case OSTREE_REPO_MODE_ARCHIVE:
-          path = _ostree_repo_get_archive_content_path (self, checksum);
-          break;
         case OSTREE_REPO_MODE_ARCHIVE_Z2:
           {
             if (self->enable_uncompressed_cache)
@@ -299,7 +296,6 @@ checkout_file_thread (GSimpleAsyncResult     *result,
   /* We can only do hardlinks in these scenarios */
   if (!is_symlink &&
       ((checkout_data->repo->mode == OSTREE_REPO_MODE_BARE && checkout_data->mode == OSTREE_REPO_CHECKOUT_MODE_NONE)
-       || (checkout_data->repo->mode == OSTREE_REPO_MODE_ARCHIVE && checkout_data->mode == OSTREE_REPO_CHECKOUT_MODE_USER)
        || (checkout_data->repo->mode == OSTREE_REPO_MODE_ARCHIVE_Z2 && checkout_data->mode == OSTREE_REPO_CHECKOUT_MODE_USER)))
     {
       if (!find_loose_for_checkout (checkout_data->repo, checksum, &loose_path,