remove "Unreleased" section from CHANGELOG (#18027)

This section is empty, and imo isn't really helpful in React's changelog. I'm honestly not sure why this is even here? Figured I'd start a discussion with a PR, or we can remove it right now.

Author: threepointone

CHANGELOG.md

@@ -1,11 +1,3 @@
-## [Unreleased]
-<details>
-  <summary>
-    Changes that have landed in master but are not yet released.
-    Click to see more.
-  </summary>
-</details>
-
 ## 16.12.0 (November 14, 2019)
 
 ### React DOM