`bp_blogs_record_existing_blogs()` is too taxing of a routine to be called from inside `bp_core_activation_notice()` (which is responsible for notices beyond just activation) so remove it for now (and refactor a bit of the preceding logic too.)

We'll relocate this function call to a more appropriate location soon. See #5749.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8592 cdf35c40-ae34-48e0-9cc9-0c9da1808c22