MDL-27814 blogs: Cleaning up contexts in blogs

All blogs pages should be in system context. Blogs are in system context just about various things (course, activity etc)
MDL-27814 blogs: Remove redundant code

Since all blog pages are in system context always, it is useless to try to get course/mod params from page context
MDL-27814 blogs: Deprecate blog_get_context_url()