Blogs: Invalidate total blog count when site is deleted.

This commit:
* Uses `bp_blogs_clear_blog_object_cache()` to invalidate the total blog count after a site is deleted and when a site is created.
* Deprecates `bp_blogs_format_clear_blog_cache()`, a function that isn't used internally as of this commit, for v2.2.0.
* Adds a unit test.

Fixes #6062.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9251 cdf35c40-ae34-48e0-9cc9-0c9da1808c22