Remove unnecessary config files and GitHub workflows (#921)

## What changes are proposed in this pull request?
Removes the following files:
* `.codegen/changelog.md.tmpl`: The new automated tagging does not use
this. API Changelogs are now generated at update time and use a separate
template.
* `.codegen/changelog_config.yml` and `.github/workflow/message.yml`:
Changelogs are now manually written and we don't need to enforce the
tags in the PR message.

## How is this tested?
N/A

## Changelog Skip
No user impact

NO_CHANGELOG=true

Author: hectorcast-db