Remove experimental Changelog file

The Changelog format is good but having a file for it is not. The same
format can be had in messages by having using vc-dir and C-x 4 a. A
temporary dummy changelog appears and that content goes to the git log
message, which is just as good in content and much better in form.
Remove an obsolete hack notice

* yasnippet.el (yas-x-prompt): Remove hack notice.