Remove CHANGELOG

Summary:
No one reads this and it is never updated. There are some real changelogs on the
Wiki:

https://secure.phabricator.com/w/changelog/2011_08_august/

Test Plan: N/A

Reviewers: jungejason, nh, tuomaspelkonen, aran

Reviewed By: aran

CC: aran

Differential Revision: 1039