Integrate gitlab-ci/remove-distro-images

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Commit: seco-ne/yocto/infrastructure/gitlab-ci@a731b156

Remove DISTRO_IMAGES

This variable has been removed in Yocto. Its contents have been merged
into DISTRO_RELEASE_ARTEFACTS.

See for reference:
seco-ne/yocto/layers/meta-seconorth-distro@a316e95e