ZOOKEEPER-2672: Remove CHANGE.txt

The CHANGE.txt is already not the source of truth of what's changed after we migrating to git - most of the git commits in recent couple of months don't update CHANGE.txt. The option of updating CHANGE.txt during commit flow automatically is none trivial, and do that manually is cumbersome and error prone.
The consensus is we would rely on source control revision logs instead of CHANGE.txt moving forward; see https://www.mail-archive.com/devzookeeper.apache.org/msg37108.html for more details.

Author: Michael Han <[email protected]>

Reviewers: Edward Ribeiro <[email protected]>, Flavio Junqueira <[email protected]>, Rakesh Radhakrishnan <[email protected]>

Closes apache#169 from hanm/ZOOKEEPER-2672-branch-3.5