Cleanup some things from blist theme removal

* Remove unnecessary loop over blist cell renderers.
* Remove `BGCOLOR_COLUMN`, which is always `NULL`.
* Remove redundant if conditions.
* Remove empty `` tags.
* Restore `name_color`/`status_color` difference in `pidgin_blist_get_name_markup`.

Testing Done:
Compile only.

Reviewed at https://reviews.imfreedom.org/r/499/