repo: Simplify internal has_object() lookup code

There was some leftover intermediate cruft here I noticed
while reviewing another patch:

 - We had an output `GFile*` for that was never used
 - We required the caller to allocate the loose pathbuf, but
   none of them ever reused it
 - We had an extra intermediate function

Also while looking at this, I'm now uncertain whether some of the
callers of `_ostree_repo_has_loose_object` should really be invoking
`ostree_repo_has_object()`, but let's leave that aside for now.

Closes: #272
Approved by: alexlarsson

src/libostree/ostree-repo-private.h

@@ -135,8 +135,6 @@ _ostree_repo_has_loose_object (OstreeRepo           *self,
                                const char           *checksum,
                                OstreeObjectType      objtype,
                                gboolean             *out_is_stored,
-                               char                 *loose_path_buf,
-                               GFile               **out_stored_path,
                                GCancellable         *cancellable,
                                GError             **error);