core: Make ostree_get_relative_object_path() private

I plan to rename all of these APIs to use the term 'loose', so that it
makes more sense after pack files are introduced.  External users
should not use them; instead use _load_variant() or _read_commit().

src/libostree/ostree-core.h

@@ -167,10 +167,6 @@ void ostree_object_from_string (const char *str,
                                 gchar     **out_checksum,
                                 OstreeObjectType *out_objtype);
 
-char *ostree_get_relative_object_path (const char        *checksum,
-                                       OstreeObjectType   type,
-                                       gboolean           compressed);
-
 gboolean ostree_get_xattrs_for_file (GFile         *f,
                                      GVariant     **out_xattrs,
                                      GCancellable  *cancellable,