Remove CHANGELOG

• Keeping the CHANGELOG up to date with every commit is a pain.
• It's impossible to get a clean cherry-pick with a CHANGELOG in place.
• Really, the CHANGELOG should be automatically generated by the packaging system based on commit logs: #1163116: Auto-generate a CHANGELOG.txt for packaged releases
• The only reason we added it in the first place was to make it easier to identify dev releases (#1040896: Fix coder review for @file headers and include a CHANGELOG.txt), but now dev releases are incremented by the packaging system, so they're easier to identify.

In summary, it's going, unless someone can provide a compelling argument to the contrary.