Remove cruft

(Like all of the patches which I'm currently posting, this is based on #1164578: Port and extend tests so that one needs to be commited first as this one mit not apply cleanly otherwise)

The attached patch removes a number of functions (some commented out) which are obviously not used (anymore).

- A theme function that is not used anymore
- An access callback function which is not used (maybe that is supposed to be used later, if so, I can update the patch)
- Commented out hook_node_view_alter() and hook_field_extra_fields() functions
- simplenews_get_categories, which is a weird duplicate of simplenews_category_list()