Remove CHANGELOG entry for PR #28681 [ci skip]

- It is present in Rails 5.1.3 already.
- Backport commit https://github.com/rails/rails/commit/9efa0b70a638dcfdc05b30ab530f89f2847bb9c2

activerecord/CHANGELOG.md

@@ -347,10 +347,6 @@
 
     *Ryuta Kamizono*
 
-*   Quote database name in `db:create` grant statement (when database user does not have access to create the database).
-
-    *Rune Philosof*
-
 *   Raise error `UnknownMigrationVersionError` on the movement of migrations
     when the current migration does not exist.