Remove clearpath_common repo.

We've eliminated this metapackage, and are now hoping to release clearpath_base separately (from its own repo, and a new release repo). When I try to do that with things in their current condition, bloom-release gives the following error:

```
Failed to open pull request: AssertionError - Duplicate package name 'clearpath_base' exists in repository 'clearpath_base' as well as in repository 'clearpath_common'
mikepurvis@testbox:~/husky_ws/src/clearpath_base
```

Is removing clearpath_common from the release.yaml all that's needed to fix it up, or will more surgery be required? Thanks.