Remove deprecated tools/enforce-tags and tools/update-docker-tags scripts (#1249)

This removes the deprecated `tools/enforce-tags/main.go` and
`tools/update-docker-tags.sh` scripts that are no longer used or
functional.

The change includes a safety check to ensure `update-docker-tags` is not
referenced anywhere else in the repository before removal.

[_Created by Sourcegraph batch change
`keegan/rm-enforce-tags`._](https://sourcegraph.sourcegraph.com/users/keegan/batch-changes/rm-enforce-tags)

Author: keegancsmith