cleanup: remove unused `.ci/common-functions.sh` script

This PR removes .ci/common-functions.sh script, unused since https://github.com/jenkinsci/docker/pull/1630.

Testing done

### Submitter checklist
- [x] Make sure you are opening from a **topic/feature/bugfix branch** (right side) and not your main branch!
- [x] Ensure that the pull request title represents the desired changelog entry
- [x] Please describe what you did
- [x] Link to relevant issues in GitHub or Jira
- [x] Link to relevant pull requests, esp. upstream and downstream changes
- [x] Ensure you have provided tests - that demonstrates feature works or fixes the issue

jenkinsci / docker

cleanup: remove unused `.ci/common-functions.sh` script #1869

Testing done