Remove `blog-id-cache` cache group

[flixos90]

The function get_blog_id_from_url() is the only function that uses the old blog-id-cache group. Since the introduction of WP_Site_Query it's basically redundant in there though, since site query results are cached anyway.

Therefore I think we should remove the cache read and write actions in that function. We can then also remove the cache group completely (from clean_blog_cache() and registration as a "global group").

[spacedmonkey]

My patch removes uses of the group in core and in tests. I have removed the global group, as plugins may be using it for something.

I believe we should be getting rid of the all cache look ups that use domain / path. I believe most domain mapping solutions out there will break the cache invalidation.

[flixos90]

Per bug-scrub discussion:

Maybe we should hold off removing that cache, although it's redundant. That is because WP_Site_Query caches are invalidated on every change to any site (via last_changed), while blog-id-cache keys are only invalidated on updates to the specific site. So the blog-id-cache keys are much more "stable".

I'm not sure whether this is actually a reason to hold off, but we need to discuss and further investigate.