ci: Remove CircleCI configuration (hashicorp#3065)

CI is now performed via Github Actions. Some notable Github Action
workflows that replace the CircleCI jobs are:

- test: replaces CCI build, test-api, test-sdk
- test-sql: replaces CCI test-sql
- make-gen-delta: replaces CCI make-gen-delta

See:
    https://github.com/hashicorp/boundary/actions/workflows/test.yml
    https://github.com/hashicorp/boundary/actions/workflows/test-sql.yml
    https://github.com/hashicorp/boundary/actions/workflows/make-gen-delta.yml
    https://docs.github.com/en/actions
    https://cli.github.com/manual/gh_run

Author: tmessi