chore: remove case-sensitive duplicate file

[glasnt]

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 clones 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)

[conventional-commit-lint-gcf]

🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

-- conventional-commit-lint bot
https://conventionalcommits.org/