Remove docker image from CircleCI config

[thinkh]

Closes #255

Developer Checklist (Definition of Done)

• Descriptive title for this pull request provided (will be used for release notes later)
• All acceptance criteria from the issue are met
• Branch is up-to-date with the branch to be merged with, i.e., develop
• Code is cleaned up and formatted
• Code documentation written
• Unit tests written
• Build is successful
• Summary of changes written
• Wiki documentation written
• Assign at least one reviewer
• Assign at least one assignee
• Add type label (e.g., bug, enhancement) to this pull request
• Add next version label (e.g., release: minor) to this PR following semver

Summary of changes

• Remove unnecessary Docker image from CircleCI config

[thinkh]

@anita-steiner Should we switch to a specific Docker version (see https://circleci.com/docs/2.0/building-docker-images/#docker-version)? If yes, which? Previously we had 17.05.0-ce-git, CircleCI default is 17.09.0-ce. The 17.09.0-ce does work (tested in ordino_product). The question is if we want to update to a higher version?