Consider getting rid of the `cloud-native-image` job

The following discussion from !71185 (merged) should be addressed:

• @rymai started a discussion: (+1 comment)

  I haven't tested this and not sure if it's actually ever used (cloud-native-image is manual for gitlab-org/gitlab and gitlab-org/security/gitlab tags? 🤔

  Yeah. This is a remnant from the past - when CNG wasn't hooked to release-tools. Added by gitlab-foss!19025 (merged), for gitlab-org/charts/gitlab#232 (closed). I think we can clean that up (maybe in the next MR).