chore: remove case-sensitive duplicate file (#13340)

The contents of `changelog.md` are a subset of the contents of
`CHANGELOG.md` in the google-cloud-orchestration-airflow package.

Without this change, `git clone`s of this repo yield git errors and
issues submitting changes (due to unexpected changes)

```
warning: the following paths have collided (e.g. case-sensitive paths
on a case-insensitive filesystem) and only one from the same
colliding group is in the working tree:

  'packages/google-cloud-orchestration-airflow/CHANGELOG.md'
  'packages/google-cloud-orchestration-airflow/changelog.md'
```

(Issue replaced on MacOS, does not appear on Debian)

Co-authored-by: ohmayr <omairnaveed@ymail.com>