repo: Remove unused remote-cache dir

Unused leftover from when ostree had pack files; minor cleanup.

https://bugzilla.gnome.org/show_bug.cgi?id=739746

src/libostree/ostree-repo-private.h

@@ -45,7 +45,6 @@ struct OstreeRepo {
   GFile *deltas_dir;
   GFile *uncompressed_objects_dir;
   int uncompressed_objects_dir_fd;
-  GFile *remote_cache_dir;
   GFile *config_file;
 
   GFile *transaction_lock_path;