fix: remove container_scanning override for multi target docker images

As noted in #182 (closed), container scanning is a bit broken ATM in any case and the current container_scanning override has no effect on the mandatory container scanning job inserted into the pipeline.

Remove it pending a better multi-image solution.

Tested in a pipeline in the account data manager project: https://gitlab.developers.cam.ac.uk/uis/devops/iam/activate-account/account-data-manager/-/pipelines/945265.

Closes #182 (closed)