remove CHANGELOG entry for Rails 5.0 only feature. #18918

`has_secure_token` hasen't been released yet. No need to track
every change in the CHANGELOG.

activerecord/CHANGELOG.md

-*   `has_secure_token` does not overwrite value when already present.
-
-        user = User.create(token: "custom-secure-token")
-        user.token # => "custom-secure-token"
-
-    *Wojciech Wnętrzak*
-
 *   Use SQL COUNT and LIMIT 1 queries for `none?` and `one?` methods if no block or limit is given,
     instead of loading the entire collection to memory.
     This applies to relations (e.g. `User.all`) as well as associations (e.g. `account.users`)