Remove container_scanning override in multi image docker fragment

The container_scanning override in the multi-image docker fragment no longer functions as anticipated now that container scanning is inserted as a mandatory container job and is no longer present in the main pipeline.

This results in "YAML invalid" errors (example).

The reason to override the container_scanning job was documented to ensure that the dotenv artifact from build is available. However this is done via dependencies and not needs and so the override is only inserting a dependency which is already implicitly present due to the stage the container scanning job runs in.

Container scanning in general is a bit broken ATM for multi-image repos (https://gitlab.developers.cam.ac.uk/uis/devops/tech-lead-forum/-/issues/34) so remove the override pending a more generic approach.

Removing the override allows pipelines to succeed (example) albeit with slightly incorrect container scanning.