core: Use openat() for reading bare file objects too

...unless we want xattrs, in which case we have to fallback to path
lookup due to lack of llistxattrat().

This looks nicer in strace.

src/libostree/ostree-repo-private.h

@@ -66,10 +66,6 @@ _ostree_repo_ensure_loose_objdir_at (int             dfd,
                                      GCancellable   *cancellable,
                                      GError        **error);
 
-GFile *
-_ostree_repo_get_file_object_path (OstreeRepo   *self,
-                                   const char   *checksum);
-
 gboolean
 _ostree_repo_find_object (OstreeRepo           *self,
                           OstreeObjectType      objtype,