Deprecate admin_created_user_subject()

When `admin_created_user_subjec()` was merged from MU, the accompanying filter was left behind. As it has never been used by WordPress core, and is not an otherwise useful function, it can be deprecated.

Fixes #29915


git-svn-id: https://develop.svn.wordpress.org/trunk@30005 602fd350-edb4-49c9-b593-d223f7449a82