More clean up in `bp_blogs_record_existing_blogs()`:

* Do not touch the `$bp` global directly.
* Remove user/subscriber intersection and use `bp_blogs_add_user_to_blog()` instead of `bp_blogs_record_blog()`. This reduces complexity and leans on a function purpose-built for doing this comparison rather than duplicating part of that logic.

See #5749.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8599 cdf35c40-ae34-48e0-9cc9-0c9da1808c22