Remove lib/post-types.php and lib/metaboxes.php

We now recommend placing custom post types and
metaboxes within a plugin that contains all of
your site specific functionality.

See the latest update to https://github.com/retlehs/soil
to find an example:

https://github.com/retlehs/soil/tree/master/mu-plugins/base

lib/metaboxes.php

@@ -1,3 +0,0 @@
-<?php
-
-// Custom metaboxes

lib/post-types.php

@@ -1,3 +0,0 @@
-<?php
-
-// Custom post types