Remove changelog header for unreleased version

We only add the header when releasing to avoid some conflicts.

[ci skip]

actioncable/CHANGELOG.md

-## Rails 6.0.0.alpha (Unreleased) ##
-
 *   Rails 6 requires Ruby 2.4.1 or newer.
 
     *Jeremy Daer*

actionpack/CHANGELOG.md

-## Rails 6.0.0.alpha (Unreleased) ##
-
 *   Rails 6 requires Ruby 2.4.1 or newer.
 
     *Jeremy Daer*

actionview/CHANGELOG.md

-## Rails 6.0.0.alpha (Unreleased) ##
-
 *   Enable select tag helper to mark `prompt` option as `selected` and/or `disabled` for `required`
     field. Example:
 

activejob/CHANGELOG.md

-## Rails 6.0.0.alpha (Unreleased) ##
-
 *   Add support for timezones to Active Job.
 
     Record what was the current timezone in effect when the job was

activemodel/CHANGELOG.md

-## Rails 6.0.0.alpha (Unreleased) ##
-
 *   Rails 6 requires Ruby 2.4.1 or newer.
 
     *Jeremy Daer*

activerecord/CHANGELOG.md

-## Rails 6.0.0.alpha (Unreleased) ##
-
 *   Rails 6 requires Ruby 2.4.1 or newer.
 
     *Jeremy Daer*

activestorage/CHANGELOG.md

-## Rails 6.0.0.alpha (Unreleased) ##
-
 *   Rails 6 requires Ruby 2.4.1 or newer.
 
     *Jeremy Daer*

activesupport/CHANGELOG.md

-## Rails 6.0.0.alpha (Unreleased) ##
-
 *   Fix bug where `URI.unscape` would fail with mixed Unicode/escaped character input:
 
         URI.unescape("\xe3\x83\x90")  # => "バ"

guides/CHANGELOG.md

-## Rails 6.0.0.alpha (Unreleased) ##
-
 *   Rails 6 requires Ruby 2.4.1 or newer.
 
     *Jeremy Daer*

railties/CHANGELOG.md

-## Rails 6.0.0.alpha (Unreleased) ##
-
 *   Deprecate passing Rack server name as a regular argument to `rails server`.
 
     Previously: